Sounds like something could be floating or picking up noise? Try measuring the voltage at the inputs of your breakout board/interface. If its low/unstable, try adding a pullup resistor.
Check that you've chosen the correct parallel port address (device manager > ports). Is it an internal or external one?
http://pminmo.com/PMinMOwiki/index.php5?title=Laptops (All I needed to do was step 1 to get mach3 driver test stable).