What a nightmare .... have you checked:
-The radiator for being hot all over. Idea here that possibly the radiator's internals are partially clogged. Use your hand (engine off) and check to make sure it is hot all over.
-Thermostat. Is it clogged or not opening fully?
-Coolant, what is your ratio in mixture? Certainly not more than 50/50 water to coolant ratio.
-Does your engine have an external oil cooler. Does this get really warm as well?
-Are all the coolant hoses getting really hot as well?
Have you posted in the Diesel Discussion Group
Keep us posted,