Why does the screen image shake?

I've got a 2007 Santa Rosa edition MBP and every once in awhile, everything I'm viewing on the screen will start shaking violently. I've tried taking a screen shot when it's happening, but the event cannot be captured. It's not happening right now, but last night it was. A restart seems to cure the problem for about 10 minutes until it starts shaking again. I'm thinking I'm in the midst of a hardware failure, but I have no idea which component it might be. Y'all got any ideas?