I don't know if this has any relation to your problem, but I've been evaluating Mach3 and I had exactly the same result you got; namely what appeared to be random deviations from the desired path . .
It did not occure in any programs that I wrote but did appear in any wizard programmed code.
Eventually I noticed that there was a pattern to it. The tool was turning WAY early like you do in your car if you were driving REALLY fast, and that led me to check into the feed rate, motor speed and accelleration settings.
I use feed per rev and the wizard put out IPM, so the wizard stuck the feed at say 50 in the code which is 50inches PER REVOLUTION in feed per rev. That's what was causing the tool to go off path in my case. I would not swear to it, but I don't think I got any errors . . . it just ran fine, but with the tool doing exactly what yours is doing.
In messing around trying to figure the problem out, I notuced that (if I am remebering right) that the accelleration set in the motor tuning also can cause the tool to meander is Mach thinks the motors need more time to accell decell than they really do.
So you might check your motor tuning settings and see if something has changed there.