I had what sounds like the same problem; when I took it to the Apple Store, they seemed pretty sure it was the motherboard. In my case it also shows up in games (especially Halo) and high-res images. I also now cannot record/input sound. I've done a complete wipe a clean install, and the problem didn't go away, so clearly a hardware issue of some kind. If anyone has a different theory, or better news, I'd be glad to hear it. On the plus side, It's continued to function, minus those issues, for almost two years now.