I'm sure you have all figured this out already but.....the above mentioned cleaning method will only work for most dirty trackball issues. I recently had to disassemble mine completely in order to clean it. there are four tiny rollers with magnets under the trackball. you can remove the bottom plastic retainer to expose and clean them. just be careful to remember how the rollers and magnets are positioned when you reassemble them otherwise the ball will track in the wrong direction. example: when you scroll up the result will be a scroll to the right
btw there was a bit of lint in the rollers that would not have been removed by the standard cleaning method.