Running 10.6.4. Occasionally, when I scroll down (using the mouse) just a little bit at a time, the screen will jump UP, before scrolling down. This happens quite frequently, but not consistently. It's incredibly annoying. I thought it was the mouse, so I replaced it with a new one (used to have a Microsoft "Comfort Mouse," now I've got the Mighty Mouse), and it's still happening. My guess is it's some kind of software issue. Any ideas?