Nick, it's pretty hard to miss the error message flashing on the mach3 screen, so presumably it's not this, but considering the changes you've made USB dropout is a distinct possibility. IIRC the spindle will stop and the reset button will be flashing in mach3 which will need to be pressed to connect back to the machine, the error message will be flashing in the little text box next to the reset button. People generally recommend not to have anything else running off that USB bus/chain thing whatever it's called, which in a laptop the two ports are almost certainly going to be running off the same board. So try running it with no other USB devices plugged in
But that said losing all that position sounds like missed steps somewhere, or at least mach3 thinks it's moving when in reality it's not, which shouldn't happen to that extent with the USB dropout problem. You will lose position with that but only by the minuscule distance it takes the machine to physically stop from whatever speed it was going, because mach3 will still think it was in exactly the same spot it was in the moment the connection dropped.
For future reference I have found that restarting mid code in mach3 can be a crap shoot and it's far more reliable to just make a new code file starting at some point not too far behind where you left off (appreciate this might not be so easy with one of those 3D wood carvings) and also jogging the machine at any time during a file will make it lose its place and it will restart relative to the position it's in when you press start. So if it paused/stopped and you jogged it over to that side a bit before continuing the file, that would explain why it went so far over there. I'm not entirely sure why this happens because in my mind it shouldn't. Maybe the restart 'safe z, start spindle, move to coordinates' thing uses relative positioning. Not sure, never looked in to it, just accepted it is what it is and remembered never to move it under these circumstances
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