As the title says: my Z dropped 4mm total in 38 minutes time. I was doing an engraving with lots of ups and downs.
Things that didn’t happen:
Things that might be a problem:
For reference, I have my Z rapids at 900mm/min, and plunge speed at 300mm/min, but more important are the acceleration rates. My Z accel is at 60mm/s^2. Thst gives the machine lots.of time to “catch” the Z axis on the way down, and then move ip upwards smoothly. (X and Y accel is set to 1500mm/s^2, IIRC.) I have the Z accel set lower than it needs to be, but this doesn’t slow down most cut jobs by too much… except maybe the cribbage board…
I had some z axis issues and I had to lube up the rails as dust was getting stuck in there. Not sure if that helps you or not though
What current do you have for the Z motors? Are they wired to their own drivers?
The fact that it is going down suggests it is skipping steps to me. The weight of the gantry always adds to the load when going up and is subtracted when going down.
Friction on the screws is a good thing to check. Same with Z speeds.
On the supply side, I wonder about your stepper drivers and current setting.
Same settings and drivers that Steven has, 1.5A because the steppers are rated for 1.7A.
I am going to turn that down to 800, I think that’s what I had for the Primo. But the gantry was also less heavy.
The Z motors are wired to their own driver? Or wired in series?
Have you checked that your bit isn’t slipping down?
Yes, as stated in the first post. 
The motors have one TB6600 each. It was the 1500mm/min, didn’t happen again with 800mm/min.