It sounds like the thermal cut out is being operated in the controller, I would have thought a fault code would have been displayed. But there is a tunnel fan located between the finger control levers and the SEMS controller and starts running as soon as the the truck key switch is turned on and dead man switch operated. It blows cool air down the fins off the SEMS controllers sometimes these fail also they can get clogged up with dust and debri. Cleaning them out sometimes gets them going again. I hope this helps.

Regards Nero.