I occasionally have sudden jumps (visible after the exposure as all stars are "doubled") in the tracking stepper. How do I fix this problem?
I have a German equatorial mount so that's what my answers are based on. If you have a fork mount, some of these aren't going to make sense.
A few things come to mind:
You might be out of balance. When adjusting your counterweights you want the east side to be slightly heavier to keep pressure on the gears. Not a lot, just enough to keep tension. Your balance might have too much or too little (or reversed) pressure. This document describes it decently.
You might need to simply wait for an hour or so. If your scope is passing through the point where the scope and the weights are vertically aligned, you may be catching it when the weight shifts from one side to the other.
A few less easily fixable things come to mind too:
Your scope could be overloaded. Make sure the mount is designed for the scope and counterweight you have on it.
I've read from a few places that taking the mechanism apart, cleaning, and re-lubricating makes a huge difference to how smoothly it tracks. Not for the faint of heart though.