z axis increases height with every pass

[cropwell]

Doddy, you are right in your first example, but if you are cutting a 10mm pocket in 1 pass you will cut an 8mm pocket and retract to Z5 and stop. but if you work the example of a 10mm pocket with 2mm DOC and 1mm stall slippage you will get a 5mm pocket and retract to Z8 at the end, but never cut air, just a shallower DOC.

Anyway. I would be interested to know if the OP saw the cutter moving around way above the original material surface, or just above the cut level. It still points to a stepper / driver wiring issue. Or maybe condensation messing up the control signals.

My experience of tool slippage is that it works its way downwards out of the collet before it snaps due to excessive forces. This has happened to me twice, and one of those times it went through the spoil board to the bed.