Ramdom loss of power

[TSPowell]

My buddy just purchased an 04. 5 auto with 120,000 miles. The truck is experiencing a pretty bad loss of power randomly. It usually occurs when its cold and the truck has sit for 2 days. We are located in Colorado and when trying to climb hills it wont go over 1700 rpm, and 40 mph. He says if he gives it more fuel it smokes a little more. I figured its probably a fuel filter, fuel pump, rail relief valve, or FCA issue. I just cant figure out why it happens when its cold. ANY IDEAS?? Im going to try and figure out the fuel pressure soon. I also have a fitting from GDP to see if the rail relief valve is leaking.