I have an idea that you might like to check out, find out exactly what current your PCB is drawing from the parallel port outputs in milliamps. Its only guaranteed to be a TTL output so when its high (5 volts) it should not exceed 2.6ma. When low (0 volts) it should not exceed 24ma..
Many commercial and home built units simply take too much current and damage the interface.....
Here is a good tutorial on the printer port:-
http://ee.cleversoul.com/parallel-port.html
You can find this at point 6. in the tutorial:-
6. Electrical

See also the tutorial section below on TTL outputs.

The Data Out pins were orginally driven by a 74LS374 octal latch, which could source 2.6 mA and sink 24 mA. There were 0.0022uF capacitors between each
line and ground to reduce transients. The manual warns "It is essential that the external device not try to pull these lines to ground", as this might
cause the 74LS374 to source more current than it could handle without damage.
Some parallel ports can supply more current, but it is almost impossible to know which one can........so always take on the "legal" maximum for safety.....
Having Windows running as well can cause problems, but as I am not a Mach3 user, take that with a pinch of salt, but you might check that you have the correct SP level for the software, that might be a problem.....
Regards
Andy