Hey mate,
I recently had this problem too. For ages my comp would drop out, usually in games like the UFO series and Jagged Alliance BiA. So annoying and weird because I can run most AAA games on full no worries. Anyway i've got a Radeon HD7700. I eventually fixed it. But there are about 50 things it could be. Here are some things I tried.
- Update drivers
- Roll back drivers (sometimes new drivers cause the problem. make sure to fully wash the newer drivers out or you'll get more problems.)
- Turn off "Windows Aero". In personalize turning the theme to "Classic" should do it
- Make sure your comps power is in "High Performance" in control panel
- AMD has the Catalyst Control Center for GPU performance changes. Not sure what Nvidia...