G'Day Ray
I just tried a few things and found something interesting. If I switch the power off to the computer and then switch on (doesn't matter if the CNC is powered on or off, load mach, go to MDI screen and type s1000 it comes back with line 2 - unspecified error down the bottom will show syntax error, on the spindle speed it will put in 1000, if you click on spindle start the spindle will start. stop the spindle and type m3 nothing happens or type m4 still nothing. So what I have to do then is close mach shutdown windows and restart load mach and everthing is fine.
You could be experiencing the same problem without knowing it because it doesn't seem to do it all the time. were you load mach run your code and the spindle does stupid things. try before you run any code just starting and stopping the spindle using m3 or m4 command and see what happens. if you get the same thing it must be a computing/program problem.
As I said earlier it wont do it all the time, when I tested then it did it 4 times in a row before I got it to work properly.