The car reaches normal operating temperature within minutes. Any temperature-based abberation is going to happen within the first half hour, although you could "inspire" that by towing something up a hill - temps rise real fast then! You'll know if it's temperature-related - your aircon compressor will shut down.
I wonder if there's something in the fuel system. Maybe a vac lock in the tank, could be a blocked breather? Try the same trip with the fuel cap left loose. The fuel system pumps diesel around like a demon - the entire 80L tank will pass through the pump every 5-10 minutes. However, if the tank is completely sealed, it should start having trouble drawing fuel after some of the fuel has been used and the pressure above the fuel in the tank drops enough. Then, the pump is challenged to draw fuel - which may start to draw air from the joins around the filter, or the primer.
Which gives me an idea. Next time the car starts acting up, immediately pull over and shut down the engine, pop the bonnet and try squeezing the primer bulb. It should take no more than 2-3 squeezes to go firm. If it goes for more, you may have a small leak allowing air into the line when the tank is reluctant to release fuel. Loosening the cap and clearing the breather will fix the problem - the leak isn't worth pursuing unless it's happening when the rest of the system is in good order.
Just some ideas, it's not a common problem!