Remove power, and temporarily swap X and Y motor cables.
Power up, and if the Y axis starts doing it (with X commands from Mach3) , the driver is faulty or current limit not working correctly.
If the X axis movement (being controlled by Y axis on Mach3 still exhibits the same problem, then the stepper motor is faulty.
It is possible stickiness int the X axis could cause it, but that is a remote possibility.
By substituting X and Y, wires, cables, PC ribbons, etc, most faults can be isolated to motor/driver/cable PC problems.
Ensure all connectors, crimps, etc are OK. Tug each wire firmly. Loose terminals/crimps/pins can cause all sorts of strange problems.
Particularly motor connectors, which can upset the current mode operation of the microstepping drivers.