What mouse is it exactly? Optical or mechanical? Wired or wireless? DPI?
How long have you used the mouse? Since when does it show the symptoms?
Does that time coincide with driver or OS install?
Do the symptoms only appear when you are actively using the mouse or directly after?
Is this in an office or home context where someone else has ready access to the hardware?
My first guesses...
- gunk if it's a mechanical mouse
- radio interference in a wireless mouse
- very high sensitivity mouse + bumps or vibrations from something like speakers
- problem in newly installed driver
- prank software
- remote control software
- guy in the next cubicle has plugged in a second mouse in your computer and nudges it slightly now and then