Optical mice are not perfect. On some surfaces the optical unit seems to think the mouse is moving when it isn't. I find both my mice (different types on 2 separate PCs) don't like my cream coloured desk surface (they are supposed to work OK without mousemats). Similarly problems arise with using some mousemats - as far as I can tell it is the pattern on the mat that the mice don't like.
Stuartli - The cursor was moving a long way (always right to left) even when the mouse was stationary.
Batch - Agreed...my mice don't like wood grain!
Rdave13 - Have done that already...and yes it's a good idea, solving my wood grain problem.
Fingees - Both original and replacement mice displayed the same phenomenon
Eric10 - You responded first, but I've saved you until last! I think that you may have solved the problem. It has been an intermittent problem, but I'm cautiously optimistic. I suspect that it's a problem which you are familiar with? Why does the 'fix' work?