For about three months now, I have been crunching with a newly bought Palit 970GTX. I set the GPU clock to around 1.360MHz, memory clock 3500MHz. Temperature never exceeded 68°C, mostly it was below. So far, all went well.

This morning, while crunching a LongRuns Gerard_A2AR_OSTRUC_LUF, GPU-Z and NvidiaInspector show GPU clock 405MHz, Memory clock 810MHz (temperature 59°C).
NvidiaInspector does not let me change the clock Settings into any direction, neither for the GPU nor for the Memory clock. The only thing that can be changed is the fan speed (which, in this case, does not help any, of course).

Is the GPU defective, or what else could be the reason for this behaviour?

Due to some computing error your GPU switched to 'safe mode'. You can restore its normal state only by a system restart.

I made a system restart already, no change though :-(

With those settings you'll have to change your power rating MAX to 120% because on those units it will go to roughly 110% of TDP

To use inspector you must close BOINC and remember to set inspector to remember settings or you will have to redo every reboot.

Then perhaps you should *not* overclock your card temporarily (and do a restart).
If it does not help abort the WU, and try another one (do a restart).

... abort the WU, and try another one (do a restart).