Although the symptoms are not completely consistent with a compression to coolant leak, I would recommend at least a compression test and coolant test for combustion gases if available.

I was a passenger in a tow truck recently that had a misfiring Ford Navistar diesel under heavy load and grades. The coolant temperature climbed through the roof (260F) and as the engine overheated, so did the intake air, which reduced the oxygen and increased the black smoke. We had to stop every 5 miles to cool down and refill the reservoir which was overflowing big time.

The head gasket was leaking combustion gases into the coolant that only became apparent under heavy load. As the coolant was heated by the high pressure gas, the symptoms worsened because the head was deforming due to heat expansion.