Help needed, Compumotor OEM650 drive wandering...

This problem has dogged me for years now, and I need to find a solution.
I'm running Ahha Artisan, driving N42 stepper motors with OEM650 drives.
Been using them for years now. The issue is that the Z axis will wander
about randomly, mostly stepping in one direction which happens to be Z+,
but sometimes will wander down (Z-). This can occur while executing a
G-code program, while Ahha is idle and doing nothing, or will happen
with the computer at the DOS prompt. Thus, I do not think it is an issue
with Ahha, but rather a noise issue.

I've never had this problem with the X and Y axis. Sometimes, certain
feedrates in circular interpolation will cause a few lost steps, but
never ever have either moved even one step unless commanded.

All three drives are alongside one another, and step/dir input lines are
as physically removed from any AC as I can get them.
I have swapped the Z with X at the connectors where power leads enter
the cabinet, and the problem moves to the X axis.
I am thus sure the issue is with the drive itself, or it's inputs.

The symptom is that the motor will creep when not in commanded motion.
That is, it will take one or more random steps at varying time intervals.
Sometimes it will dither back and forth and more or less maintain
position, but usually it will creep in one direction. I put a dial
indicator in the quill drive housing just to track this. I can generally
tolerate a thou or two drift during a single part, just correcting and
rezeroing Z between parts.

I have a shop built rotary phase converter near by, on the floor. Got it
from a fellow that had vast experience in high voltage electrical stuff,
it's given not a lick of problems in the near 30 years since he built
it. Can't say that it's radiating any noise, no way to test. I have
plugged a Radio Shack line filter between the drives power supply and
the mains, to no effect.

It would seem that maybe this particular drive is overly sensitive? All
three are configured with identical settings.
So, I'm asking for suggestions on how I might solve this. Could running
the step/dir input lines through a ferrite torroid a few wraps help?
Anything I might install inline on these lines that might help filter
noise without affecting signal timing?

Haven't looked for an OEM650 drive on eBay for a while, but it's been a
long time since I saw one for sale. Going to start looking again and
swap out if I can find another. For the work I do, if I have to live
with lost steps, Z is the one to lose them in, so can't swap with X or Y.

Thanks,


Jon