Just a thought: I know my work laptop (which has one of those "nipple" things) occasionally drifts sideways as if there is a slight offset pushing it. Assuming you use a separate mouse for normal day to day use (as I do, for convenience) try disabling the built in mouse. I'm not sure how to do this (but know it can be done) - I guess its in the Control Panel + System + Hardware area.
This way at least the problem can be narrowed down.
Good Luck,
FujiF