Ok well first turn the Kernal speed sown to 25000hz you don't need it that high. Going to high here stresses the parallel port.

Next the fact you are testing on the bench and not fastening the motors down will cause resonance which could be some if not all of the problem.

Also the Steps per setting how did you come to the figure of 320.? . . . . And What are Micro steps set to on your drives.?

Are each axis set to the exact same step per figure and tuning.? You must select the Axis on the side bar to correctly display for that axis and then save after every change.