z axis increases height with every pass

[cropwell]

If the stepper were stalling on plunge it would be taking shallower and shallower cuts until it only just skims the surface.

I would check that you have solid electrical connections to your Z motor. An intermittent connection to a phase can produce some weird motion.

[Doddy]

If the stepper were stalling on plunge it would be taking shallower and shallower cuts until it only just skims the surface.

...resulting in the Z0 position effectively rising, and subsequent plunges being positioned higher than expected.

EDIT: But the advice on checking wiring is worth following through.