Just throwing in my WAGs
what about checking the plugs and pins connections (particularly the inputs and can bus connections between the controllers).
Does it do it only while driving on the floor or will it do it if you have the drive wheels off the floor?
Have you taken a eyeball and confirmed ALL the drive motor brushes, and have you checked 'shorts to frame' on all the power cable connections?
What are the codes you are seeing? (?only watchdog?)