Don't know what was causing the scaling, even if I commanded a G50. I updated the postprocessor, so the max feed and rapids doesn't cross my machine limits.
Mach3 is working OK now.
Thank you for your help Overloaded.
Now, I'm fighting against another problem. My router doesn't accept feedrate updates during gcode running. Neither feedrate overwrite.
Some times, in the gcode, there are some Fx... commands,like ones on toolpath linkage, and my machine servos jerks when these commands lines runs.
As if they were jammed or something, giving just small "punches" instead of the right movement.
Any clue how to solve this issue?