You could have something, dirt or lint stuck in your trackball area. Do you carry in your pocket, this could be the cause. Best course of action is to buy a case. You may also get lucky and find a respected repair shop/dealer who can clean it without voiding your warranty. Hope this helps and feel free to PM me with any other questions.
Also if you are out of warranty, or really brave you may try this, but be aware it could void your warranty if you have one:
Reddogs how-to:
http://blackberryforums.pinstack.com...ictures-38395/
Hope this helps . . .