In order of importance:

  • Run the game with your computer cabinet open to mitigate possible overheating.
  • Clean inside your cabinet. Dust may be preventing your fans from turning.
  • Test your memory with Memtest86 from http://www.memtest86.com/
  • Test your CPU with Prime95 from http://www.mersenne.org/freesoft.htm
  • Update your drivers.
  • Reset your BIOS settings to factory default.
  • Reset your BIOS settings to safe mode, if possible.
  • Open both cabinet and use a window/fan/whatever to really make sure overheating is not the issue.
  • Swap around internal power cables. Could be the one that goes to your GPU is overloaded and doesn’t feed enough power to maintain it.
  • Could be your PSU is generally overloaded. Unplug any device that you are not using. If you have 2 HDDs, remove the excess one. If you have any unused PCI cards, take them out.
  • At this point, pray. It could be anything and you need to do an individual component check, but it is most commonly the PSU that’s the culprit.
  • …if all that fails, report back when you find what exactly was wrong.