I'm not the most experienced here with touchplates, but sounds like the probe connection is screwy.

Do you have the touch plate type with a switch in or is it just closing the circuit on contact?

If its the switch type, the behavior you're describing would tally with the switch not activating in some cases (the machine keeps going down until it sees the switch close), or not releasing in other cases (the machine is supposed to hit the switch, then back off until the switch opens again - but never sees the switch open again so carries on up).

Might also apply to the closed circuit on contact - but either way I'd be looking at my zero plate and it's wiring rather than the Z motor.