could be a number of things... if its an optical mouse make sure you have a SOLID mousepad.... mousepads with white/black/letters/pictures are horrible for optical mice. So go with a solid black/white/blue whatever..

could be a hair on the bottom of your optical mouse, could be software. In game there is an option to use Hardware cursor... try that. Just try a couple of things... I myself had this issue once and it was a hair at the very tip of the optical laser on the bottom of the mouse... blew it out with some canned air and never had that problem again.