Perhaps left field - but check the water pump plate
	
	
		I had a ford 250 and there was a steel plate between the aluminium pump body and the block. Don't know if this applies to your engine though. The steel plate had rusted out on mine and had a smallish hole, less than a 5c piece. As a result alot of the water wasn't going through the radiator, lots of cavitation I guess. This showed up when the engine was at highway speed. You'll see what two gaskets under the pump, with the steel plate in the middle.  Only real way if it is rusted is to pull the pump off.