Folks, I've got a sick Mercedes on my hands

Car in question is an 84 300D Ran like a top until work was recently done on it. It had all the air and fuel filters changed, an oil and filter change and a valve adjustment and now it runs like pure hell.

SYMPTOMS:

It will start fine and idle well at about 650-700 RPM while cold, but starts to run worse as the motor warms up. If you take it on the road cold, it runs ok until it warms up, then it runs worse. When I mean running worse, I mean once warmed up it will stutter/miss at about 2000 RPM, and bog down if you try to put any load on it. Then in about another minute or so the motor slowly loses RPM no matter how much throttle you give it. Eventually it gets so bad you are going about 10 MPH with it floored, and if you take your foot off the accelerator the motor will die. You can re-start it but it idles bad. It will slowly rev up, but as soon as you engage the transmission, it chugs and tries to die.

If you let it get warm in the driveway it runs smoothly while cold, but as it warms up the idle starts to become VERY rough and will drop to 500 RPM and the whole motor shakes like a paint mixer. You can try to rev it but if it builds RPM it does so slowly and chugs badly. like it is only running on 3 cylinders. If you are able to get it above 3000 RPM , it is rev happy and smooth, then you let off the throttle and it instantly dies.

Let it sit until cold and start it and it is fine until it warms up, then it runs like hell again

In order to trouble shoot it, here is what I have done. Changed the primary fuel filter with a new one again. No change. New air filter, no change. Cleaned the tank screen (not dirty at all) and blew the tank feed and return lines clean. No change. Removed the lines from the IP and the secondary fuel filter housing and blew them clean, no change.

Lastly I tried to isolate the fuel system. I hooked up a 2 gallon fuel can to the primary filter and then used the return line from the secondary fuel filter housing to go back to the 2 gallon can. On the road it ran better but still had the 2000 RPM stutter/miss. Idled better too but still started to run worse as the motor got warm, just not nearly as bad or as fast. Back home I re-hooked up the regular fuel system, and it ran a lot worse and almost died out and stranded me.

So, what the hell could have caused the problem(s)? I'm down to maybe a bad secondary spin on fuel filter (even though it is brand new) or a screwed up valve adjustment. After that, I'm out of ideas.

Based on what I described, what would you do. HELP!!

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)