With respect to the drive issue. If the car has been running well, check tach for excess AC ripple. Check armature for grounds and proper brush tension. I have seen the current sensor zero pot drift.
That drive has very few regular adjustments. You have a gain pot and some jobs wire in the IR comp ckt. Normally you need to modify the RC time constants in the speed loop and current loop to adjust it if the gain pot doesn't work. But that would not be the case on an existing unit.Make sure you are not seeing the current limit led flicker on at top speed. Again indication of loop grounding on an existing job, or even brake dragging. The null forcing pot can cause problems if drastically misadjusted. IR comp can help dampen small vibrations. Again look for more obvious things, ie tach ripple, tach brushes being short or excess carbon.