You didn't say whether the wastegate is closing or not. My 87 had similar symptoms as yours and I found there was the crud built up around the flapper in the wastegate (the gate?) and it was binding - never fully closed- lots of effort to clean it paid off, I was about to send the turbo for rebuilding when I found the problem. 257K miles young and still strong!
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!