MY buddy owns a stock 04 cummins 6-speed manual and its running rough. When he starts it in the mornings, it'll idle about at 450 or 500 rpms until it jumps to high idle at 1000. Then driving, it seems like it doesn't have all the power it used to, kinda lugs. when he slows down for intersections it will drop down to 450 to 500 rpms like its going to die, it has died a couple of times, so I'll step on it, rev it up and the rpms will then bounce between 500 and 900 and after while it'll even out back to 750. I changed my fuel filter about 4 days ago. I haven't check my fuel pressure yet. And I don't receive any codes on the dash either. Any suggestions? Bad injector, lift pump?
I just dont know I'm not a CR guy yet

 

I think checking fuel pressure would be a good start. It would be good advice to pass along to him to install a permanantly mounted Fp gauge to keep an eye on things. Low FP can lead to some expensive repairs on these things.

 

I own an 06, Cummins 6 spd but don't live in a cold climate. A little more information would be helpful, e.g., Did the problem appear suddenly? Has the source of fuel changed recently? Is the fuel in the tank old? Has the vent in the top of the fuel tank been checked?

BTW, I bolded the text in your post because it is really strange that you start out talking about your "buddy's" truck and then in the 4th sentence you suddenly change from talking in the 2nd person to lst person.

Good luck to whoever's truck is having a problem.