Doubt it is the actual mouse itself, as the mouse cursor only moves in response to a wheel rotor breaking to light beam. Does it still happen when the mouse is unplugged.
My guess is some dirt etc inside the keyboard. Pull it apart and clean all contacts.
You could even try buying a new keyboard - about $10 is all.
Reckon the above will fix the problem as I can't see it being anything but a hardware prob.
