This problem reoccured when I had my oil and filter changed at a fast lube. When I pulled out of their parking lot the engine had about 50% of normal power. I watched a mercedes mechanic a month ago tear off the metal cap on top of the fuel injector pump on the back right of the engine and open the set screw. This helped at the time but I tried to adjust it out even more a couple of days ago with no response. Thanx again for your help.

The set screw on top of the injector pump was the ALDA. If the Mercedes mechanic adjusted the ALDA, he was smart enuff to check the Boost Line and Banjo fitting for plugging? I had an 87 300D where the boost line was plugged and I didnt lose 50% horsepower!
I'm not a guru, I'm a gorilla who asks questions from those who are smarter. I still kind of think the problem to be the fuel pump. Even if the white boost line gets plugged, you won't lose 50% horsepower. I went thru this, I changed injectors, filters,everything but in the end it was the pump.
You can buy a used one for $150.
It may be worthwhile to take your car to a good Mercedes independent in your area. Someone on the forum can recommend one to you.