I've the same mouse. Had the same problems when I wasn't using a mousepad. I game alot so it finally dawned on me that the surface under the mouse had become polished. An optical mouse sometimes has problems with shiny surfaces. I got a mousepad and the problems went away....well, for awhile at least.
I'm now noticing irratic behaviour with the mouse...sometimes when I drag objects, the cursor doesn't want to let go of the object. Other times, if I'm trying to grab a window to drag it somewhere on the screen, the drag registers as a double-click and the window ends up maximizing.
I love the mouse itself. The problem may be because, as I said, I game....ALOT. My mice usually accumulate mileage very quickly in a short amount of time. It could be that I just need another mouse.
Also, this particular mouse doesn't seem to respond well to KVM switches. I lost all the cool features of this particular mouse when I started using KVMs. I've read about this problem in reviews also. Sorta sucks, but I bear with it. Problems like, the scroll wheel stops scrolling and the side buttons stop functioning also. I've noticed that this only happens when I've recently rebooted and haven't pressed a KVM button yet. As soon as I go to another machine via the KVM then come back, the functionality is lost. What's funny is that this doesn't happen on the Linux boxes...only the XP ones (driver issues?).