Cursor creep

...sometimes when I'm kicked back from the monitor and my Compaq optical mouse from Radio Shack is sitting quietly on the tabletop, my hand nowhere near it (and no earthquake occurring that I'm aware of), the arrow cursor on the screen just starts migrating slowly upward and to the left at about a 45° angle. It'll stop for a while, then start up again. It works just like it's supposed to when I put my hand on it (though fairly regularly, without my making a move of any kind, the pointer will jump to the lower left edge of the screen or sometimes the right center and I have to go fishing for it and bring it back). Anybody ever experience anything like this? Any clues?

Thanks,

Bill Schuhle