Sounds like you need to bring someone in and have the servo(s) dialed in on a Ball-Bar. Never seen one go out that far but there are parameters that control the motor/screw movements. Is the problem just in certain areas of the machine or doesn't matter where? Does it change if you slow down or speed up? Does it change depending on the size of the arc? The answers to these may help to isolate the problem.
The issue with Nervis... I've seen that before too on the older controls. I always believed it to be comp related in some way. Never pursued the issue much though I'm almost always programming in G-code anyway (for mills).
It's just a part..... cutter still goes round and round....