No Power when towing

[2000 7.3 powerstroke]

My uncles 2008 6.4L f350 feels like it has no power and the key light comes on in the dash when he tows an empty horse trailer and the problem starts when he drives for around 2 hours and when he goes up hill, so far he has had the problem 2 times. He never has the problem when the truck is empty or not towing anything. He recently had to get the fuel injectors replaced because they got clogged. He took it to the dealer and they couldn't find anything.

Anyone got any ideas?