The problems that you are all encountering fall ininto three main catagories:

1] Bearing surface design,both of you have too small of a bearing surface to structurally hold your instruments

2] Gear engagement and worm block design. Rebuilding a marginal design does not improve a thing. A redesign is in order.

3] Gear quality, pitch angle, smoothness. tooth count all contribute to the precision of a drive system. Most likely, your drive train has periodic errors that will limit you, as well too little or too much contact pressure between the gear sets.