This could be a problem with your servo, the servo amplifier, or the sensor chain. This is an easy problem to troubleshoot. Do you have a voltmeter that can measure resistance? A DVM? If you don't you can get on from Radio Shack for $40 or so.

If your thumbwheel is bad that would be very easy and cheap to fix. We have a rebuild kit on our website for $40.

If the thumbwheel and sensor chain are OK then it's a problem with the servo or the servo amplifier. If either one are bad you'll probably have to replace both or better yet dump the servo and go with the upgrade.