Forklift at work...
I tend to agree that the source of the problem is not the connectors. It sounds like you have an issue in your robot.
The only way I can think of their being an uneven current flow is if you have a short through the frame to ground. Also, if one connection starts to fail before the other, then the resistance on one side will increase at a higher rate than the other. This could cause uneven heat production. Seems unlikely though.