[Frank7]

The abrupt shut down sounds like a lose or broken wire, or a short cell in one battery. The batteries are the place to start.

Check to see if the motor terminals have any heated areas or any movement at each terminal.
These are the areas I would choose to focus on way before the controller.
The tow run switch isolates the electronic area and that is what leads me toward the batteries and motor.

Good luck and be careful.