Computer problem?

My computer has been on the fritz for the last few days. I downloaded nVidia nTune to keep things running well, but ever since all my games play for about ten seconds, than stop for 2 or 3 seconds, and then plays for ten seconds, stops for 2 or 3......etc. I've uninstalled nTune and run a complete virus scan, but found nothing. Wtf is going on? My computer is still messed up. Help please!