I had a mouse that had that problem. Did it on my G4, and on the dual 500 to a lesser extent.
The problem manifested itself SOMETIMES in the Finder, but always in Unreal Tournament - after differing amounts of time. It'd just die.
Unplugging then replugging worked in the Finder, but but not in UT. Which makes sense, so thats ok.
But yeah, I replaced the mouse with an identical one like it, and, that works fine.
My God I hate USB. With a passion.