Beware of home brew remedies. Engines can have hot (under pressure) leaks and then cold leaks (ambient pressure). A tiny suction leak (air) between radiator and water pump can cause the most hysterical, impossible to diagnose overheating symptoms you can imagine.
I am going to recommend yet another path to try...
The dealer. The manufacturer of the chassis. Surely the Parts Department counter guy has run into this and knows the best way to resolve it.
My next trip to the states I am going to buy a STANT cooling system pressure tester kit. It would have found my nightmare suction leak in a few minutes.