Check the obvious first, like cleaning the rollers the ball turns.
Then make sure that the computer is not just busy doing something at the moment the mouse hangs, so what you are experiencing is your PC ignoring you (sorta), not a hardware problem, just a momentary lack of resources.
See if there is harddrive activity during these episodes. Do you run something in the background like Napster, or even Microsofts' FindFast, or are you on a network and someone else is accessing your PC ? Any of these can cause short periods of rigor mousis.
Hope it helps
Ray