I'm encountering problems with Mach3 producing erratic tool profiles.   when it should be making a contouring pass inside a rectangular area, moving like your eye would read a page of text, the limits of travel do not meet the edges of the rectangular box consistently.  I listened to the motors and they accelerate and decelerate smoothly, with no jerks or discontinuities of the glissando ramping pitch of step-pulse-rate,  so it's not an acceleration issue.  I also hear no jerks or discontinuities during the smooth portions of each axis's motion.  I increased the step pulsewidth in motor tuning to the maximum of 5us in case that was too short,   from 4us,  but no change in behavior there.

What's especially odd is that,  though the cutter doesn't traverse the full width of the rectangular region when it's experiencing problems,   it will often *return* to the correct margin of the box after several passes (eg. lines of text read, in analogy) with different alignment / margins / width of cut. 

I'm using Mach3 version R3.043.066 on Windows 7 Professional SP1 on an Intel Core2 Duo running at 3ghz.

Any help you can offer to resolve this would be most appreciated.
 

have you tryed to run the Programm without spindle ON in the air?