Have 2004 Diesel 3500. Twice in the last few years, truck has lost power while at freeway speeds and climbing hill. Each time, truck suddenly slowed to 25 mph but continued on. There were no warning lights or any other sign of problem. First time, dealer diagnosed bad fan clutch sensor and apparently the engine went into low RPM mode to save the engine. Second time, they found nothing wrong in engine diagnostics.

Anybody else heard of this? Any thoughts?

 

Sounds like lack of fuel, Im sure you have changed the fuel filter recently, also the new ultra low sulfur diesel fuel is pretty crappy not as many btu's

 

might get some ideas in the cummins section. Sounds like lack of fuel like force55555 said. the lift pump might be going out.

 

Engine might be going into "limp mode" for some reason. Overheating will cause it to limp.