Windows 10 64bit, GeForce GTX 960 last driver.
GPUGRID works on 0.003% per second but after 1:30 min without touching the keyboard or moving the mouse, it always slows down to 0.001% every 5 seconds but the task is still active.
If I move the mouse or touch a key, the initial speed is restored.
I have tried everything as to make the screen always active, many different configurations of the Boinc client. The strange thing is that it only happens with GPUGRID, with MilkiWay GPU does not occur. Nor with other CPU projects.
I have had to put a program called Stims that simulates sending a key every minute.
What can this be?

Well ... Can you share the contents of 2 files, please? :
- global_prefs.xml
- global_prefs_override.xml

Also, the first 50 lines of Event Log might be useful.

<global_preferences>
<source_project>http://www.gpugrid.net/</source_project>
<source_scheduler>https://www.gpugrid.net/PS3GRID_cgi/cgi</source_scheduler>
<mod_time>1500744504</mod_time>
<run_on_batteries>1</run_on_batteries>
<run_if_user_active>1</run_if_user_active>
<run_gpu_if_user_active>1</run_gpu_if_user_active>
<idle_time_to_run>1</idle_time_to_run>
<suspend_if_no_recent_input>0</suspend_if_no_recent_input>
<suspend_cpu_usage>0</suspend_cpu_usage>
<leave_apps_in_memory>1</leave_apps_in_memory>
<cpu_scheduling_period_minutes>60</cpu_scheduling_period_minutes>
<max_cpus>0</max_cpus>
<max_ncpus_pct>100</max_ncpus_pct>
<cpu_usage_limit>100</cpu_usage_limit>
<disk_max_used_gb>0</disk_max_used_gb>
<disk_min_free_gb>50</disk_min_free_gb>
<disk_max_used_pct>0</disk_max_used_pct>
<disk_interval>60</disk_interval>
<vm_max_used_pct>100</vm_max_used_pct>
<ram_max_used_busy_pct>100</ram_max_used_busy_pct>
<ram_max_used_idle_pct>100</ram_max_used_idle_pct>
<work_buf_min_days>0</work_buf_min_days>
<work_buf_additional_days>0</work_buf_additional_days>
<confirm_before_connecting>0</confirm_before_connecting>
<hangup_if_dialed>0</hangup_if_dialed>
<max_bytes_sec_down>0</max_bytes_sec_down>
<max_bytes_sec_up>0</max_bytes_sec_up>
<daily_xfer_limit_mb>0</daily_xfer_limit_mb>
<daily_xfer_period_days>0</daily_xfer_period_days>
<dont_verify_images>0</dont_verify_images>
</global_preferences>

<global_preferences>
<run_on_batteries>1</run_on_batteries>
<run_if_user_active>1</run_if_user_active>
<run_gpu_if_user_active>1</run_gpu_if_user_active>
<suspend_cpu_usage>0.000000</suspend_cpu_usage>
<start_hour>0.000000</start_hour>
<end_hour>0.000000</end_hour>
<net_start_hour>0.000000</net_start_hour>
<net_end_hour>0.000000</net_end_hour>
<leave_apps_in_memory>1</leave_apps_in_memory>
<confirm_before_connecting>0</confirm_before_connecting>
<hangup_if_dialed>0</hangup_if_dialed>
<dont_verify_images>0</dont_verify_images>
<work_buf_min_days>0.400000</work_buf_min_days>
<work_buf_additional_days>0.000000</work_buf_additional_days>
<max_ncpus_pct>100.000000</max_ncpus_pct>
<cpu_scheduling_period_minutes>9999.000000</cpu_scheduling_period_minutes>
<disk_interval>60.000000</disk_interval>
<disk_max_used_gb>0.000000</disk_max_used_gb>
<disk_max_used_pct>100.000000</disk_max_used_pct>
<disk_min_free_gb>0.000000</disk_min_free_gb>
<vm_max_used_pct>100.000000</vm_max_used_pct>
<ram_max_used_busy_pct>100.000000</ram_max_used_busy_pct>
<ram_max_used_idle_pct>100.000000</ram_max_used_idle_pct>
<max_bytes_sec_up>0.000000</max_bytes_sec_up>
<max_bytes_sec_down>0.000000</max_bytes_sec_down>
<cpu_usage_limit>100.000000</cpu_usage_limit>
<daily_xfer_limit_mb>0.000000</daily_xfer_limit_mb>
<daily_xfer_period_days>0</daily_xfer_period_days>
</global_preferences>

23/07/2017 12:37:50 | | Starting BOINC client version 7.6.33 for windows_x86_64
23/07/2017 12:37:50 | | log flags: file_xfer, sched_ops, task
23/07/2017 12:37:50 | | Libraries: libcurl/7.47.1 OpenSSL/1.0.2g zlib/1.2.8
23/07/2017 12:37:50 | | Data directory: D:\2 Programas instalados\BOINC\Datos
23/07/2017 12:37:50 | | Running under account i5
23/07/2017 12:37:51 | | CUDA: NVIDIA GPU 0: GeForce GTX 960 (driver version 384.76, CUDA version 9.0, compute capability 5.2, 4096MB, 3414MB available, 2618 GFLOPS peak)
23/07/2017 12:37:51 | | OpenCL: NVIDIA GPU 0: GeForce GTX 960 (driver version 384.76, device version OpenCL 1.2 CUDA, 4096MB, 3414MB available, 2618 GFLOPS peak)
23/07/2017 12:37:51 | | Host name: i5e
23/07/2017 12:37:51 | | Processor: 4 GenuineIntel Intel(R) Core(TM) i5-6500 CPU @ 3.20GHz [Family 6 Model 94 Stepping 3]
23/07/2017 12:37:51 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss htt tm pni ssse3 fma cx16 sse4_1 sse4_2 movebe popcnt aes f16c rdrandsyscall nx lm avx avx2 vmx smx tm2 pbe fsgsbase bmi1 hle smep bmi2
23/07/2017 12:37:51 | | OS: Microsoft Windows 10: Professional x64 Edition, (10.00.15063.00)
23/07/2017 12:37:51 | | Memory: 15.96 GB physical, 18.34 GB virtual
23/07/2017 12:37:51 | | Disk: 931.51 GB total, 388.33 GB free
23/07/2017 12:37:51 | | Local time is UTC +2 hours
23/07/2017 12:37:51 | | Config: don't use GPUs while GTAVLauncher.exe is running
23/07/2017 12:37:51 | GPUGRID | URL http://www.gpugrid.net/; Computer ID 433068; resource share 99
23/07/2017 12:37:51 | Milkyway@Home | URL http://milkyway.cs.rpi.edu/milkyway/; Computer ID 736161; resource share 0
23/07/2017 12:37:51 | Rosetta@home | URL http://boinc.bakerlab.org/rosetta/; Computer ID 3224902; resource share 0
23/07/2017 12:37:51 | World Community Grid | URL http://www.worldcommunitygrid.org/; Computer ID 3684175; resource share 100
23/07/2017 12:37:51 | GPUGRID | General prefs: from GPUGRID (last modified 22-Jul-2017 19:28:24)
23/07/2017 12:37:51 | GPUGRID | Host location: none
23/07/2017 12:37:51 | GPUGRID | General prefs: using your defaults
23/07/2017 12:37:51 | | Reading preferences override file
23/07/2017 12:37:51 | | Preferences:
23/07/2017 12:37:51 | | max memory usage when active: 16344.30MB
23/07/2017 12:37:51 | | max memory usage when idle: 16344.30MB
23/07/2017 12:37:51 | | max disk usage: 389.19GB
23/07/2017 12:37:51 | | (to change preferences, visit a project web site or select Preferences in the Manager)
23/07/2017 12:46:27 | GPUGRID | Sending scheduler request: Requested by project.
23/07/2017 12:46:27 | GPUGRID | Not requesting tasks: don't need (CPU: job cache full; NVIDIA GPU: job cache full)
23/07/2017 12:46:30 | GPUGRID | Scheduler request completed
23/07/2017 12:56:09 | World Community Grid | Sending scheduler request: To fetch work.
23/07/2017 12:56:09 | World Community Grid | Reporting 1 completed tasks
23/07/2017 12:56:09 | World Community Grid | Requesting new tasks for CPU
23/07/2017 12:56:13 | World Community Grid | Scheduler request completed: got 1 new tasks
23/07/2017 12:56:15 | World Community Grid | Started download of MCM1_0134765_1400_MCM1_0134765_1400.txt
23/07/2017 12:56:17 | World Community Grid | Finished download of MCM1_0134765_1400_MCM1_0134765_1400.txt
23/07/2017 13:11:22 | World Community Grid | Sending scheduler request: To fetch work.
23/07/2017 13:11:22 | World Community Grid | Requesting new tasks for CPU
23/07/2017 13:11:27 | World Community Grid | Scheduler request completed: got 1 new tasks
23/07/2017 13:11:29 | World Community Grid | Started download of MCM1_0134765_5685_MCM1_0134765_5685.txt
23/07/2017 13:11:31 | World Community Grid | Finished download of MCM1_0134765_5685_MCM1_0134765_5685.txt
23/07/2017 13:15:29 | World Community Grid | Sending scheduler request: To fetch work.
23/07/2017 13:15:29 | World Community Grid | Requesting new tasks for CPU
23/07/2017 13:15:32 | World Community Grid | Scheduler request completed: got 1 new tasks
23/07/2017 13:15:34 | World Community Grid | Started download of MCM1_0134765_4392_MCM1_0134765_4392.txt
23/07/2017 13:15:36 | World Community Grid | Finished download of MCM1_0134765_4392_MCM1_0134765_4392.txt
23/07/2017 13:33:41 | World Community Grid | Sending scheduler request: To fetch work.
23/07/2017 13:33:41 | World Community Grid | Requesting new tasks for CPU
23/07/2017 13:33:45 | World Community Grid | Scheduler request completed: got 1 new tasks
23/07/2017 13:33:47 | World Community Grid | Started download of MCM1_0134598_8300_MCM1_0134598_8300.txt
23/07/2017 13:33:49 | World Community Grid | Finished download of MCM1_0134598_8300_MCM1_0134598_8300.txt
23/07/2017 13:46:31 | GPUGRID | Sending scheduler request: Requested by project.
23/07/2017 13:46:31 | GPUGRID | Not requesting tasks: don't need (CPU: job cache full; NVIDIA GPU: job cache full)
23/07/2017 13:46:34 | GPUGRID | Scheduler request completed

I'm not seeing anything odd here, so far.

Next step -- Does GPU-Z's "Sensors" tab indicate that GPU Load (and other characteristics) actually change behaviors, when running "0.003% per second" versus running "0.001% every 5 seconds"?

I mean, if GPU-Z shows that the GPU is being used the same, then this simply might be some obscure bug in the GPUGrid application's ability to report progress correctly. A screenshot of GPU-Z showing both behaviors, would help here.

GPU load: 93% -> 100%
Memory controlled load: 52% -> 83%
Bus interface load: 27% -> 2%

Can the gpu work so hard that the percentage is not updated correctly?

For the next test ... close BOINC.
Then monitor GPU-Z, again, to see if something weird happens after 1:30.
Again, please provide another screenshot.

Maybe you possibly have another program using the GPU, like a Bitcoin malware or Folding@Home or some other app, where ... when you press keyboard/mouse, it "hides itself", thus restoring GPUGrid performance.

Another thing to monitor would be Task Manager, sorted by CPU descending, to see what apps are using any CPU after 1:30.

Yes, it is confirmed that it is another program that works at the same time on the GPU. After closing Boinc in 1: 30m puts the GPU to work until I move the mouse.
In the Task Manager, ordered by CPU, the program does not appear nor hear noise from the system fans.
Thanks for the help. I will follow the track of a bitcoin type trojan as these are the only ones that could use the GPU.

You're welcome. When you find more info about the culprit, please also add it to this thread, I am curious and it could help others. Thanks!

Yes, it is confirmed that it is another program that works at the same time on the GPU. After closing Boinc in 1: 30m puts the GPU to work until I move the mouse.
In the Task Manager, ordered by CPU, the program does not appear nor hear noise from the system fans.

Thanks for the help. I will follow the track of a bitcoin type trojan as these are the only ones that could use the GPU.

Problem solved.
It is a Trojan that went with The SIMS 4 Vampires. It is loaded at startup so with disabling its load, it is fixed.

taskhost.exe (taskhost_300.exe)
Analysis: https://www.virustotal.com/es/file/6464a73c600ac8b080c0606072608f84159a6e345b43c98f0f72a23d7b571df7/analysis/

Location: C:\Users\<user>\AppData\Local\Microsoft Windows\taskhost.exe
Date: 22/12/2016 Size: 96KB
Registry: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
Name: Host Process for Windows Services

Thank you very much for everything.

Problem solved.
It is a Trojan that went with The SIMS 4 Vampires. It is loaded at startup so with disabling its load, it is fixed.

taskhost.exe (taskhost_300.exe)
Analysis: https://www.virustotal.com/es/file/6464a73c600ac8b080c0606072608f84159a6e345b43c98f0f72a23d7b571df7/analysis/

Thank you very much for everything.