Are you overclocked? That is a common symptom.
Also, try disableing stuff in your tool tray to the bare minimums and see if that makes a difference.
If not, I suspect a hardware problem. Try re-seating your cards and if you have two sticks of memory, try one for awhile. While you are in there make sure your fan is working properly and your cpu is seated tightly. Vacuum the dust out (if any

).
Also, if you can try borrowing another power supply to try for awhile.