60 Series Detroit help needed

[CSnyder]

Ok this could be a long ordeal but Ill keep it short for now.

I have a 1998 Freightliner Classic with a 60 series Detroit that feels very sick right now. It all of a sudden started loosing large amounts of power a few monthes ago to the point it works real hard to pull loads averaging gross of 75,000. I have had lots of work done on it at the local Cummins dealer and it still isnt coming around and Im sick of forking out huge money for these dumb mechanics. It has a 13 speed and even in a slight head wind or pulling an empty aluminum trailer I cant hardly ever run higher than 11th. This same truck used to pull 75,000 in any condition in 13th and if you didnt watch the speedo you would be pulling 90 or more MPH!

A Few things Ive done so far...

1) new head
2) new injectors
3) new fuel lines, filters, flush on the system
4) new ECM
5) new air filters
6) new air to air
7) tried another used ECM out of a good strong truck
8) new turbo

I can pull 30 PSI boost all day long with it but it struggles to keep speed now. Even when I bobtailed home this morning 30 miles it just felt sick. An example would be comparing a stock Dodge Cummins 5 speed and an identical truck running 400 HP plus. You can tell the difference! One other thing Ive been noticing is it is only averaging 3.53 MPG down from 7.42 since the symptoms appeared. Getting going in all the lower gears it feels like it has strong torque and power but higher gears its like someone bolted a 2x4 under my skinny pedal.

This Classic long nose is my favorite truck and used to be the strongest in my line but Im just about ready to retire it now so any ideas or help will be GREATLY appreciated!

TIA, Chris