My dad is driving a 2001 3500 dually. When he is towing a load, last night it was a 16 ft trailer with a skid steer on it, eng temps are getting hot and he says it seems like it is cutting out. He has to vary the speed to find a point that it won't do it. Already had the thermostat replaced. Any suggestions would help. Thanks

What is the eng size? Does it have the towing package?

He has the 5.9L cummins and no tow package.

I know its sounds simple but has he checked the air cleaner. Less air would be more heat. Mine plugged with snow over the winter and ran awful.