Hi MarkT,I see... The problem probably came from something else...When you say "over-driven", are you saying the stepper motor is being over-driven?The stepper motor is burning hot, way too hot to be touched. If you are referring to the stepper motor, it probably be the case. In this case, can it be fixed?
If not, then you have a mechanical problem. Disassembly of big motors is quite easy, you may find that the field windings have broken loose and are able to rotate, although that may be accompanied by a lot of noise while in motion. It may also be a single coil that is defective. This may not be noticed if positioning is via some form of encoder rather than absolute stepping, only when holding would a problem manifest itself.