Some type of malware could be causing this, but it could also be overheating.
Have you cleaned inside your computer lately? Be careful when doing this and usa a ground strap to prevent damage via static electricity. Make sure all the fans are working and not making any funny noises (case fan, CPU fan, and power supply fan).
If there's no problem there, it could be the power supply going bad. If you have another one available, try putting it in and see if it helps. If you don't have one handy, I'd try the malware stuff first.
If you suspect some type of virus, go to this thread and follow all the advice given:
http://www.daniweb.com/techtalkforums/thread5690.html
If you still suspect intruders, post a hijackthis log (explained in that thread) in the Security forum.