I am going to just throw in a blanket response here.
We have a 6040 CNCEST parallel (TX14175 BOB) and started to have Z problems intermittently.
At some point I discovered that the stepper wire on the gantry was janky; you could wiggle the wire
and the stepper would change tone. We cut out a section of the black 4-wire where it doubled back on
the drag chain, and spliced in a proper section and it fixed it. Then it started to do it again about a week later.
On closer inspection, I found on of the four wires from the motor itself where it went into the connector was
hanging by a thread because it had been over-crimped at the factory, so only one tiny filament was making contact
and that was actually separated and only mechanically touching the crimp of the pin.
We replaced the motor connector with a proper Molex 4-pin aviation connector and no problem.
Later it happened again, this time on the gantry axis, and when continuity testing with a multimeter, we found one of
the runs (on the gantry (Y)) had one wire broken inside 'somewhere'.
That was the last straw, so we replaces ALL the wire runs from the controller with PROPER aviation 4-C shielded wire
from the controller to each motor AND replaced ALL the remaining motor connectors to get rid of the HORRIBLE Chinese wire.
There was more to the troubleshooting nightmare, including swapping stepper drivers, AND purchasing a used TX14207 stepper driver
off eBay because we 'thought' the stepper driver was the fault (it was not).
So sharing this lesson before you tear all your hair out in futility.
-Christian