Ouch!! It's been awhile since looking at Mach, but I would bet that your Z axis stepper resolution settings are wrong. They (Avid) have probably changed the pitch on the Z axis ball screw or motor since you bought your machine.
David F might be a good source as I believe he is running Mach 4. If no one else chimes in with a quick response, let me know and I would be happy to try and help you figure it out.
I would bet that the counts per unit value in the Motors Config tab are exactly half off.
Last edited by beltramidave; 11-03-2019 at 07:22 AM.