I would try a block test (search "block tester" at www.napaonline.com
I know the ford diesel head gasket issues are when you lean into them, the head bolts actually stretch out and hello problems, then you let it cool down and she runs like a champ.
There is a problem here... Hopefully you can find it.
Is the radiator actually full of coolant? Some people look at the reservior for "coolant level" but it is not uncommon for a failed head gasket to drink all the coolant in the cooling system and not suck coolant from reservior.
Also are you properly bleeding the cooling system? (sorry if this has been asked in the last 3 pages)