First thing, check the coupler is snug. Second thing, **** your pants because it's probably electrical
What are you running mach3 on? Some people have problems with their PC or laptop unable to keep a constant stream of code buffering, and the machine stalls momentarily once it finally picks up again.
Does it make a horrible noise or does it just quietly go off course randomly without warning? I have had both things happen to me, the horrible noise one was due to a crap power supply. I've had it randomly go off course twice both times near the end of long jobs, don't know what caused that.
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