PWM bumped to min I think is for spindle so shouldnt affect an axis, then again I have never had spiundle control via PWM so may be off track with the message.
Because you have made the problem follow the wiring then it has to be from the drive up to computer that is the problem. Do you use a breakout board? If so see if you can use two other outputs to see if it cures it. Also check your parallel port cable and make sure it is not damaged.
Hood