I got my MX500 a few days ago (using PS/2 port), and am using it with 3M mousepad that I had. Sometimes I've noticed that the cursor starts jittering and freaks out. It has problems tracking movement. This lasts about 15-20 seconds, and then it returns to normal. I thought it was the mousepad, because my 3M has a very dark picture of "space and stars" on it. I put it on my desk (wood-grain surface), and still it jitters sometimes. My desk is kind of shiny, so maybe that is the problem. But what's strange is that the mouse will start working for a long time, using the 3M mousepad, and only very rarely start jittering. Then it always fixes itself. I may have to go get a light-colored mousepad.