1st off it's a 04 freightliner century. Mercedes engine
My shop mechanic swears it's in my head and won't even drive it with a loaded trailer. I came out of a 500hp Cummings, that's why he thinks it's in my head.

My check engine light comes on a few seconds after accerlation and goes off a few seconds after the pull is over or I let off the accelerater. It's done this since I got it 1 1/2 years ago. The power loss has been over the last few months.
It's never had descent power.
I've dropped down to 6th and 7th on mountains and 8th on small inclines.

i cant stand that motor. did you check the main fuel line to the fuel rail? they leak (badly) but at first they do what your describing.

if that 430 merc motor i had lots problems with head gaskets ,otha engines stop engine or check engine light been fuel related probs

Did you check the code for the check engine light?

go back to the cummins and don't even go near another truck that says mercedes on the door.

Was the Cummins you were in EGR? My Cummins doesn't feel like it has anywhere near the 550 hp it's supposed to and behaves like you describe sometimes...especially on warmer days. Do you get the "popping" out the exhaust? All part of the beauty of EGR.

Its not showing a code. I guess because it goes on and off.
As for going back to the Cummings, the trucks that company had was awesome but the pay and the boss blew.
Also my Cummings also made that popping back sound when you let off the accelerator quickly.
There is a fix for that but I forgot what it is.
Kevin Rutherford gets that question all the time on his radio show. Seems like everyone new truck does it.

Is this your truck? There is a code if your engine light comes on. The way to check it is to turn your truck off. Then turn the ignition but don't start the truck. Hold down the Override button for 5 seconds. The check engine light should start blinking. If it blinks 2 times and pauses and then blinks once. Its a code 21. 3 times and then 2 times quickly it's code 32 etc., etc. As for what the codes mean, I can't list them all here, but I googled "Detroit Engine Series 60 DDAC Codes" and printed my sheet out for myself.

I have the same year and model truck, my engine is a detroit, same thing almost..Sounds like the same problem i'm having with my century. Found it to be a code 39(EGR VALVE). Replaced the valve and still having problems, found a leak in the egr exhaust canister and it's now in the shop being replaced, hopefully this will fix the problem. My truck has 900,000 plus miles on it. I'm not expecting to get great power on the hills, but I would like to get better fuel mileage than 5.5 @ 65mph. Come to think of it my truck was blowing black smoke as i got down the road for about 1.5 and no one could figure out the problem. replaced the valve and it has stopped blowing smoke all the time just on shifts. Hoping thing will fix my system. although i've heard that this egr engine is terrible and the system may go out again.
But I would say have your shop check the egr system, I was told that it could be cleaned out and this may help your problem, if not it need replacing. Best thing to do is replace the whole system if you have the money, this way you know nothing is wrong with it.