I'm surprised there's no reply here.
I will chime in with limited value probably.
I have only had a shutdown like that once with the smooth stepper and it was my fault. I accidentally allowed a powersaver mode to startup on the laptop.
I took a chance despite the Mach3 warning message and hit run after the pc was back online. Tore a nice line before I could hit stop. Never again.
I know this shutdown thing happens though it has not been a problem for me.
Thoughts:
1) No unnecessary software or programs to run on the Mach3 computer. No firewall or antivirus on mine. NO POWER SAVE MODE.
2) rule out electrical interference (keep ethernet cable isolated).
3) rule out poor cable connections or jiggle
Beyond that, I dunno....
Steve