There is a guide somewhere in the forum on how to take dissaemble your trackball and clean the contacts allmost like cleaning a old mouse. You carefully dislodge the silver ring and can clean it. Just do a little search.
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Same thing happened to me a week ago. Like the Professor said, take out the silver trim ring and the trackball assembly will come right out. Roll it around a little, look for and remove any dust you see, and pop it back in. I fixed mine in about 10 minutes. The "how to" thread is here
Get a new tracball. I got one from amazon. arrived in three days. on my 8830 the silver ring pops off and the new ball comes with a new ring also... just pop that off and lift your old one out, or turn your phone down and it will fall out.... drop the new one in and snap the ring back down... that smoothed mine up greatly... however the main reason i got a new one was because mine would no longer "enter" or "click" anything. It didnt fix that problem.
I think the cost was like 5 bucks plus like not much shipping cost at all... came in the mail...