Hank The error is too consistant to be noise issue.! . . .It's repeating 0.0075.

I presume you have done the obvious checking binding, slipping couplings etc.!

This sounds very much like pulse timing issue.? basicly it's not completeing the full step before it starts the next step.!

Before doing anything else I would check simple things like wire connections, make sure the wire polarity of the motor phase's are correct regards drive connections IE: A+ B+ A- B- from motors go to same on drives. They will still work ok if switched and it can cause silly issue's like this with some setups.

If the USB Software as a similar Dir/step pulse timing setting that mach does then I'd try increasing this time.!
Back off the velocity/Acclereation and see if it still happens, you could just be over tuned.?

Are 100% sure the motor is Man enough for the Job.? Some times if the motor is on the edge regards holding Torque they can "spring" steps while going slow or near stop, again this would do this kind of repetative error.!

Can you try swapping the Axis to see if it moves or goes away.!

If you have one I would even try swapping the USB cable, I have had silly things like this with dodgy PP cables. I had a small noise issue with my smooth stepper and one of the tricks or work rounds was to remove the shielding at one end.!! . . . .Basicly the shielding at each end was causing ground loop problems. I know it sound like a long shot but worth a shot.?

hope you find it quick I know how these things can drive you crazy . . . . . . . Keeps us informed.!