Whew! I can breathe again.
It sounds like you found that information a little useful.
I almost didn't post... I'm a noob... I know it... and I'm not looking to start posting lots of useless noob advice.
I find the problem you describe very interesting.
It is very strange that it worked for so long, and then seemed to change when you tweaked some settings.
It would seem to me, that it's one of two things:
1. Either the Step signal coming out of your LPT port (if you are using an LPT port) or out of your motion controller, has changed. Meaning it's not putting out the same number of steps for a given distance as it was previously.
Or...
2. The number of step pulses coming out are correct... but somehow the stepper drivers are no longer operating in the same mode (Full, Wave, Half, some level of micro-stepping).
I'm an electronics guy... so my next move would be to hook up either a counter or a logic analyzer to my STEP signal for the problem axis... tell Mach3 to do a move that would generate a specific number of steps... and see how many pulses I actually got.
At that point... I would know something... as I would actually see the expected number of pulses... or not.
If the number of pulses were right... I'd look for what changed downstream... probably the mode that the stepper driver is running in (some can be changed).
If the number of pulses were wrong... it's got to be something in Mach3... almost certainly some configuration setting.
edit --- If you are using an external motion controller... it could also be that, as that's what would be generating the step pulses.
My take on this would be that the Mach3 software has worked for too many people for too long to have that fatal of a flaw... where it's flaky and only works sometimes... it's got to be something more definitive.
Your own attempts to solve the problem by re-installing and using different P.C.s seems to indicate that whatever it's doing... the result is very consistent... it's just not resulting in what you expect.
I wish you the best of luck.
Best regards,
Michael