I'd start by turning off the backlash compensation in Mach3. It often causes more problems than it solves. If the problem persists, try turning everything off, waiting a bit, then switching the leads from the problem motor to one that's been working okay. Switch the ports and pins designations to fool Mach into running it as the axis you switched it from, then do your test again. If the same thing happens, it could be the motor's fault or a mechanical issue, but if the problem transfers over to the new axis, then it's something upstream.
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