Gaming performance decreases mysteriously?

Ey guys,

When I start a game (DiRT 2, Burnout, Flatout UC or TMU) the FPS is about 50-60. After a while (about 5 minutes) the performance instantly drops, decreasing FPS to 15-20 and lowering. After a while the performance drops again by half. I also noticed a glitch in the sound and a delay when pressing a button. eg, when I press the “right” key to steer right, it is a fraction too late… And that’s not cool if you’re going 80 MPH and you can’t steer normally…

I’m running programs like GameBooster and Cacheman to speed things up. It is noticable, but it doesn’t prevent the mysterious performance drop… I also noticed that there are like 15 copies of the svchost.exe process running…

Thanks in advance