[MarcL]

99% likely you are losing encoder feedback from the Z servo motor to the PC console.

Could be a failure in the motor itself; could be a loose connection or damaged cable between the motor and the console.

My guess is that it is the encoder on the motor itself. Presuming your control is newer than 1996 or so, the CPU board would detect a bad cable or connection by the loss of the encoder differential channels and report "bad encoder connection". More likely the encoder's line driver is working, but its optical reader isn't.

If your motors have military-style connectors and you have enough cable, you could swap cables with another axis (e.g. put the X motor cable on Z, and the Z motor cable on X). Then you could see if the problem stays with the motor, or stays with the cable/drive/console/software axis.

Obviously, you want to be pretty cautious and keep a hand on the big red button any time you are experimenting with a machine that is inclined to run away....

Incidentally, you can see the raw encoder counts, even when you have not homed and don't have numbers on the DRO, by viewing the PID Configuration screen: F1/Setup, F3/Config, "137", F4/PID. Encoder counts are displayed in the "Abs. Pos." column in the lower center of the screen.