Have you measured the boost pressure? It should be at least 8 psi or more under load. 11 psi better. Insert a tee in the vac line and bring the gauge out into the car to check it while driving.
Also is there a Nasty Trap Oxidizer on that car? if so it may be the culprit!

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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!