I have a 3 XXL, and ever since I’ve had it, it’ll randomly decide the Z axis should be about 6mm too high or too low (and dive into my material). I’ll stop the job, re-zero the Z axis, run the exact same file, and it’ll work fine. Any ideas?
Check your belts as well if you’re on the original belt drive Z-axis. Troubleshooting similar accuracy problems on mine over the last two months, I found 2 stretched belts (the X axis and one on the Y axis, replaced all), 4 v-rollers that lost contact, bad bearings in my router, and worn nuts on the Z leadscrew. So, lots of deferred maintenance .
But what yours sounds like is the pulley slipping on the motor shaft. Mine was doing the same thing on the Y axis and was hard to diagnose. What happens is those grub screws look like they are snug, but don’t have enough force to hold. They slip one direction and catch against the flat spot on the shaft. They hold there for a while until there is enough force to slip the other direction and catch on other side of the flat spot. You end up with this random slip a few mm in each direction.
That’s why I switched those all out to socket head screws with a touch of low strength thread lock. You can put a little more force on them than a tiny grub screw and the thread lock holds them there.