I have the same type of problem IF I edit the code or jog around to set up a work piece before cutting,, What I have found is that
I will do a dry run (without the CNC active) in MACH do my editing after watching a new program run ,THEN close the G-code file ,Then
close MACH , And pull up and all will run fine , but if I edit the code or change ANYTHING I will shut everything down and re-start or it will crash.
I have had this problem from the start and seemed to have got worse after using the smoothstepper . Dont get me wrong I love the smoothstepper
but I make sure to re-start after any type of change to the program.. Oh and also mine will show the DRO going to -.750 without that even being anywhere in the code file..
Just my 2 cents worth..
Gary