Well theres ur problem right there anways not to sure why this happens, because i have a cheap wired optical too and it does this. it would probably be best to return it (if its new) and get a more expensive mouse.
what surface are you using it on??
try it on a different surface before taking it back...it may just be reflacting off of something.
also try using a different mouse (non-optical) and see what happens..
i was using a almost vinyl type mouse pad but since i moved my computer im just running it on semigloss kitchen cabnet wood and its only gone wierd once now im just using my mouse pad as an arm rest cause otherwise my arm falls asleep cause of corners jamming into my arm should i switch back to my mousepad?