If you would do a search, you would find a possible solution. http://www.blackberryforums.com/gene...mittently.html
Do you carry it in your pocket? Most likely there is lint or dirt clogging one of the contacts. Turn the device off. Press and hold the trackball and roll first in the direction of the problem and then roll it in all directions for a minute or so. Turn the device back on and it should work. You can also spray some "canned" air around the ball while pressing it down. Will help clean it.
Once you get it working, carry it in the pouch, not in your pocket.