I could not find info on this unit, so am assuming it is electric not gas as you checked for a grounded element. Usually a G in the Model Number is for gas.
Perhaps the high limit thermo?
Possible but I doubt it, the unit should be able to run on high heat without tripping the breaker as long as the breaker is the proper size. Only way it could do this is if when it opens it shorts to the frame.
I would pull the heater again and recheck it for any arc/weld marks on the inside of the case.
Could be the heating element is heating up, sagging and touching the case.