if the trackball is truly munged you can order a new one for abt 9USD$ from
Cellular Nationwide Network - 3G, Sony, Panasonic, LG, SonyEricsson, Nokia, Tools, BlackBerry, (remember to get the 2USD$ tool with it...) There is a VERY clear video on youtube on how to change it that is very good.
Also an alcohol swab across the trackball can help rid it of oils (like hand lotion) or even some link that can foul the sensors.