Most likely a Grounding issue, if your PC is desktop or tower, the PC power supply common is at the same potential as your spindle (neutral), if it is an AC motor or DC SCR controlled from the 120v mains supply.
I would guess that the majority of 'noise' problems is the lack of proper grounding of the parallel port and the service ground to a common point in order to create zero potential difference between the two.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.