OK...so I've tried the motor on a different drive - no joy. Then tried a different motor on same drive and still have the problem. I only have 3 motors, all bought at the same time. Will try the 3rd motor tonight but am not holding much hope...

I've played around with various settings (acceleration, max velocity, etc) but had no joy there, either. I then tried changing the gear ratio in software to 6:1 and had no problems as the motor runs faster even at low feed rates. The question is - should the motor be smooth throughout the range or will there be inherent "bad" spots due to the Chinese drives and motors? I'm convinced this is a stepping issue that causes resonance but only at low speeds. If I increase the step rate to 3200, the drive sounds much "cleaner" but I get a "realtime" error in LinuxCNC. What step rate is "normal" for this hardware?