Sometimes I'll leave my mouse alone while watching a movie, and I'll notice the cursor start to move by itself, yet no one is touching my Mouse, and the Mouse isn't even moving. I use the HP mouse that came with my PC with the red light.
Maybe on a bit of an uneven surface?
Cheap mice, especially optical, seem to do this quite often really. If it's annoying enough to give you trouble with screen savers or full screen video, get a replacement/RMA, very easy if you bought it at a store, they'll usually let you have whatever you want, within reason, or they'll just ship you one if you bought online.
I've seen this happen with an optical mouse that I use at work.
Mouse completely still, and the cursor will just start jittering diagonally, really slow across the screen..
I switched it at one time with the other pc I have to use at work, and it's definately the mouse. It did the same thing on the other pc.
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