Have you got "Absolute" selected in the Mach3 postprocessor configuration? If it thinks circles are relative, you can get errors like that.
Another thing you can do is to stop letting it make arcs (G02,G03) and have it output everything as line segments. It makes the code a little longer, but you don't get hung up in the IJK stuff.
Andrew Werby
www.computersculpture.com