[aiharkness]

FYI so you'll know what to expect, those symptoms will recur, if you experience is anything like mine, and will become more frequent, and take longer and longer rolling the trackball like you did to fix it. I put up with it as long as I could, and eventually repalced the trackball assembly. Afterward I didn't know why I waited so long. So, be thinking about it, and don't keep putting up with it longer than you should. (Actually, your wife may say, "Honey, will you fix this thing this time?")

EDIT: I think what happens is partly what Dubdub says, dirt and crud build up in the assembly. But I think pieces parts in the assembly also just wear out.