I had the same issue with my machine about a year ago. Turned out the Z axis stepper motor had gone bad.

I spent hours on the phone with Tormach before they found the problem.

At first, they replaced the control computer (I still use Mach 3). That didn't change anything so the next thing was to replace the stepper motor. That took care of the problem.

The smartest thing I have done is I kept up the warranty on my machine. Yes, it costs me $450.00 per year, but when I had my Z axis problem the first thing they replaced was the control computer. That was almost $800.00. I don't know how much the stepper was, but I'm sure between the computer and the stepper the bill would have been over $1,000.00.