Why does my computer lag every 5-10 minutes?

It's a recurring issue that occurs mostly when gaming. Everything runs fine for 5-10 minutes but it's like clockwork, eventually my computer lags to death for a few minutes and I can't do anything, then it slowly picks up and starts working again. By lags to death I mean my frame rate goes from 130 slowly but surely down to like 1 frame every 5 seconds then back up. I'm running Vista 32 and I recently even did a reformat on my hard drive. The problem is not malware or spyware related, to start that off. I'm very good in that arena. I have a feeling its a heating or OS issue but I'm trying to pinpoint it.