In case you haven't, it is important to check if it is a centering problem or a triggering problem. Even in auto or free run it is still triggered, so if it is the problem could be all the way back at the ramp generator.
If the trace itself is moving and not the sweep (I hope you got that...) then I would more look for hash on a PS line somewhere. Of course that applies to the ramp generator as well but different PS legs of course.
Jeff Urban