Cursor moves erratically even when no mouse connected

I've been having the issue for over a year now, but never got round to reporting it. From time to time, my cursor will move erratically around the screen, jumping position every few seconds. I have a MBP, but often use it plugged into a screen and mouse etc with the lid closed. The problem persists whether setup like this, even when the external mouse is disconnected, or when simply using it as a laptop. I recently wiped the hard drive, and reinstalled a clean copy of OSX from scratch (not from backup), yet the problem persists still.

Any ideas?