Nope.. the only changes I've made since I bought it a coupla years ago was to upgrade to Snow Leopard. Until then I had no problems. I was thinking it might be the video card as well but I wish I could confirm it before spending $500 on a new one.
The thing that confuses me is the fact that it only happens when I'm watching a video or running a screen saver. If it was the video card, you'd think it would keep happening constantly no matter what program I was running.