Something else to try. Attach a meter to the base of each Z transistor (one at a time) and set it to DCvolts. Put the machine in step and step each transistor at least 4 times. They should turn on and off crisply. If you have a bad diode on that board on the bottom right of the side door, one transistor will be off, turn on half way, turn on fully, turn off half way, You may or may not have this problem. This has happened to me only a few times in 25 years. But it can cuase symtoms such as yours.
Another very remote possibility is a flaky optocoupler on the output of the ZDI. I had one machine do this. It sends the commands to the SMD.

George