The only settings that may affect the trackballs scrolling ability is in "Options > Screen\Keyboard". In this menu at the bottom under "TrackBall" you will seen Horizontal and Vertical sensitivity which needs to be set at 70. If these options haven't been changed just restart your device by removing and replacing the battery. Let us know how you go.
i have the same problems on my 8820, its funny i never get it on my curve, but it happens all the time on my 8820.
rim's solution was to turn the phone off. press down the trackball hard and roll it in the direction u are having trouble with. to dislodge anything stuck in the trackball.
theres a q&a on the rim website, dont have it on me right now.