Ok so, a couple of weeks ago i changed my stepper driver to a totally different one that was in it.

Yesterday I tried doing a halftoner tool path (loads of z movements)
Halfway through i notice hey this looks abit deep here. So i checked and i had lost like 18mm. So ok i zero and try again. Same thing happens.

Run a profile cut today on some small items and i lost 2mm

I have tried decreasing acceleration and velocity on the z axis in Mach 3
This didn't work.

Checked all set screws and bolts on z everything is tight.

Is this just totally the wrong driver or what is happening here it is driving me bonkers please help.