Two thing's I would do. One.
Since it is somewhat new take it to the nearest provider store and have them take a look at it. With any luck they will replace it. NOTE: IF you have a warranty USE IT. Two.
If you're comfortable with taking gadget's apart, I'll give you a link and some instructions on taking your phone apart and maybe giving the trackball a good cleaning. (This may not be the problem, but if I were to take a guess its the contact under the trackball either has something on it, or it's just not making a proper contact.)
NOTE: This void's any warranty's you have on the phone, so if you have a warranty, USE IT.
Originally Posted by salracing
This issue is more apparent when pressed a bit harder than usual.
Then again, this just might be your problem, period. hahaha