Hello! Sorry you're having these problems. I've had very hot motors before (smell of melting insulation hot), with no ill effects, but anything is possible.
Let me just run through a couple of things
You switched the motors and the cables, and it's always the motor that's on the left-hand 'channel' that cuts out? That's good, I think you're right that it means the motors and cables themselves are ok.
If so, my first guess is the driver or the wiring / assembly is duff on the left side. One thing to try might be to switch the motor drivers boards themselves around. Taking the drivers out and re-seating them will do no harm either. If the right-hand side motor starts cutting out, then it's definitely a faulty driver. If the left hand motor continues to cut out, then it might be a driver, or it might just be a faulty polarshield. A little bit of corrosion or oxidisation building up over the months.
I am positive that it will be nothing directly that the controller is doing - in the sense that all the controller can do is generate simple commands - there isn't even a command for 'release motor'. It is possible that a particular move causes a divide by zero fault, or something like that (though very unlikely), but again, there is no area of logic in the firmware where one motor is released independently of the other, so even if it were being triggered accidentally, it'd be both, or nothing.
It sounds like a short to me. When I forget to insulate the top of the USB connector on the arduino, then that is the same symptom (ish) - intermittent power loss.
It's interesting that it is the same point where it cuts out. Any chance you could try turning the speed down, and see if it's the same point in the drawing, or the same time from start that is key?
If you can confirm to me that both motors and cables seem to be working, I can try to scrape together a pack of spares for you, but I'll contact you privately about that.
thanks for your patience
sn
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