My mouse pointer is bouncing around the screen on its own - why?

I've solved it. It's fair to say that my office is in a bit of a mess at the moment whilst we redecorate another room. I have a Wacom Bamboo tablet which I hardly use these days. It'd fallen down the back of the desk and my laptop PSU had landed on top of it.

I was going through the unplug-everything-one-by-one routine. I'd already tried unplugging the mouse as soon as the problem manifested itself but that didn't help. I was going through the USB cables, unplugged one and it stopped. When I plugged it back in, it started again. I followed the cable and found the long-forgotten tablet, with the PSU sat on top of it. As soon as I moved the PSU, it stopped. With a bit of trial-and-error I found that it didn't need to be in contact to cause the problem.

Anyway, thanks for the suggestions.