By any chance do you have a dust boot that moves with the Z? I had an issue of Z slip because my dust boot would foul on deeper plunges. I went through the whole gamut of trying to test different things, including the fact that my garage was about 28F degrees at the time, thought maybe the belts were hardening. Anyways, if you’ve checked set screws, checked belt tension, check V-wheel adjustment, and the only thing left is a dust boot that moves with Z, might be worth a try? I considered trimming the dust boot bristles to thin them out, but in the end went with a Suckit boot and that issue went away.