CFandZ, Quite interesting. I have had the same problem since 5/2020 when I bought the machine. Some of the following notes may be helpful to you in diagnosis.
Here is what I discovered:
Stepper motor with factory damaged winding, I took it apart and saw it.
Swapped motors and the problem went from Z axis to the X axis.
Then the problem started again on the Z after getting new parts.
Last night it started again on the Z.
Each cycle has resulted in Avid giving me new parts. Since it is intermittent, assurance of a fix is uncertain. They gave me: 1) new control box, 2) new drivers, 3) 2 new motors and a cable
It is hard to be certain what is up, I know moving a defective motor moved the problem (along with seeing the winding damage plainly).
Feel free to follow my document on the malfunction, here:
https://docs.google.com/document/d/1...it?usp=sharing
The document reads from bottom upward chronologically. I welcome any conclusions you may infer after comparing the events.