I've noticed that when I pull up a steep hill or a heavier trailer that when I step on the pedal the tranny downshifts and the motor seems to just rev and not gain rpms or vehicle speed. If I keep the converter locked it's ok but still seems low on power sometimes. If my fuel pressure is being drawn low during these times would it not set a code? If not then I guess I need to look at a new pump kit. Sometimes my cummins just doesn't pull like I think it should. The most I pull is 6000 lbs. I just thought the ecm would set a code if it saw that low of pressure but maybe it's just low enough to cause power loss. It's an 04 with 100,000 miles.

 

Other than maybe slipping the clutches (auto) I wouldn't know, do you have any gauges? IMHO, when you're pulling with a CR, it's good to have gauges especially EGT, trans temp, and at least lift pump pressure. On CR engines, rail pressure is really important and those gauges are pretty cheap now (check Quadzilla) It's definitely possible to burn your engine up towing heavy up a hill, possibly even stock and it'll seem it's running good right up until it shuts down. JMHO.
Maybe your fuel filter needs changing?

 

The guages will be coming. I'll check the fuel filter too as I guess that could starve me of fuel.