Computer Stutters!!!

Hello wonderful community.

Periodically my computer will start jittering (freezing for a millisecond) at a regular rate of every .4 seconds. It effects audio, video and my mouse (although audio is the most audible because it mutes). But this happens when only a few basic apps are running, AND it didn't happen when I successfully ran Prime95 for over 10 minutes while running StarCraft 2, chrome and my VoIP. However in the past there has been a larger chance of it happening when I launch new processes.

I'm running all the latest updates, and other than this my system has been running smooth. I am not overclocking. In fact my 1600 DDR3 RAM appears as 1066.

Specs: i7 920, 12GB DDR3 Corsair XMS3, GA-X58A-UD3R, MBox 2 Pro, 5.5TB WD Black, GeForce GTS 250, Windows 7 64bit, Antec EarthWatts 750W

Ideas? I don't know where to start!

Thanks!!
Z