I just received the MX-500 as a gift, and I'm having the EXACT same problem. Seems to work fine for a little while, then it will start freaking out, moving erratically, skipping, etc., and then goes back to normal. I exchanged it for another one, and its doing the same thing. When I put the mouse on a blank piece of paper, it seems to work perfectly fine, however, it doesn't scroll anywhere near as smooth as on my mousepad. My Logitech Dual Optical never had a problem, and yes I've got the latest Logitech drivers installed. I can't believe Logitech doesn't have this problem stated anywhere on their website, and I haven't read about it much anywhere else. I guess despite the "new and improved" optical lens/engine on the MX-500, its considerably pickier about the mousepad its used on.