A new optical mouse won't set you back very much money but if you'd rather spend your cash on something else there's bound to be life left in old rolly for a little while yet. Sounds like dirty rollers, as you move the mouse around your desk / mouse mat, tiny bits of dirt get picked up on the ball and transferred to the rollers, where they accumulate and make the mouse move unevenly. Take the ball out, and you should see two rollers. If they're dirty they'll have a big buildup of gunk usually centred around the middle of the roller. Pick it off with anything you like (I've used tweezers, fingernails, screwdrivers etc) then give it a bit of a wipe over with a damp cloth or similar, re-ball your mouse and you're set to go.