Intermittent choppy graphics with games

Hi there, new to the site.

I'm having issues with choppy gameplay on my PC. Here's the weird part though. I'm only getting it intermittently. For instance, I turn my computer on, launch a game like "C&C Red Alert", and notice right off the bat with the splash screen that the game will be choppy. So I esc out back to windows and restart the game, and it'll be fine. But when I exit and restart the game for the third time, it'll be choppy again. So it's pretty much choppy every other time I run a game. Not just C&C but every game I tried did the same thing. The only things I tried was reinstalling the game to another drive, swapping out the memory with my other PC and constantly updating to new video drivers; all with no luck. The computer was built about 6 months ago, and it's been happening from the very start. Here are my specs.

HD - 160gb WD Raptor
MB - Asus p5n-t deluxe
CPU - Intel Quad 9450
GPU - MSI 8800gt
Memory - 4gb Kingston HyperX ddr2 800
OS - XP Pro SP 3
Case - Antec 12 hundred with a gazillion fans blowing in and out of the case
PSU - 750w corsair

Everything is kept stock with no OC'ing of any sort

Any suggestions you may have will be greatly appreciated. Thanks