Loss of power

[Fate fog]

I have an 03 2500 that has 160,000 miles and is starting to loose power. Any suggestions on where to start. I was thinking that the valves need adjusting because I have not had this done yet. Doesn't Start as quickly as it used to either. Any suggestions r appreciated

 

[Prairie Dog]

Classic injector issues especially at that mileage but I would start by re-torquing the cross over tubes. If that doesn't help you could have a shop do return rate and balance testing.
Any codes stored?

 

[HBarlow]

Owner's manuals suggest valve adjustment at 150k miles but Cummins valve trains don't wear enough to make adjustments worth the trouble or expense. Valve adjustment is not causing your problem.

A competent dealer service tech, if you can find one, can run a cylinder contribution test with the in house computer to determine if all cylinders and injectors are in tolerance.