Is there any way to introduce boost into the cooling system other than a blown head gasket? Intake gasket possinly? Truck runs, and idles fine, and cooling system works normally for the most part. However if I hammer on it. I can blow a new cap of a new degas bottle. I get no water in oil, no oil in water. No excessive vapor clouds. Only other symptom is to want to start running hot up an extended grade with big trailer. She has gotten hot a couple of times, but never had a problem like this. I have been a gas engine mechanic all my life, and have never run across these symptoms.