Are you running the latest drivers from nVidia's website?
Did you have an ATI card previously?
Anything running in the background when your gaming, Antivirus, Windows updates ETC...?
The core clock in GPU-Z seems fine, its just using its "Turbo-boost" technology to overclock itself to improve performance.
Yes Im running 300.10 which is the latest and I had 2xAMD 6950's before, i do have antivirus as well but none of that caused any issues until today