I had a similar issue. Take the threaded rod off and roll it on a table, looking for run-out. That said, my issue ended up being an open connection between U1, pin 18 and J2 pin 4. This is the Z driver side on the G-shield and the Z wiring connector. I had also tightened the ACME rod Nyloc too tight causing run-out.
Look through my thread.
Here is a link to the G-shield schematics…Z axis driver is U1 on page 3.