Mach3 using the parallel port for threading had issues for some and worked great for others. With external devices it has always worked well as far as I know, certainly does with the Smoothstepper.
I think the problem most had with the parallel port and threading was down to one of two things (or both), one is a severely underpowered spindle for the threads they are trying to cut and second is if they have a VFD that is set in a mode which tries to keep the spindle at the commanded speed. What happens in that case is the spindle slows, Machs driver sees it and slows the axis but at the same time the VFD sees it and speeds the spindle, so Mach sees the speed increase and... well an ever ending loop of bouncing back and forward
I never had any issues when I was using the parallel port but then again my spindle could take all I could throw at it and never blink.
Hood