Double-check to make sure you are getting full throttle at the IP, by having someone hold the pedal to the floor for you.

I was told my a long-time Mercedes mechanic that a throttle adjustment problem is one indicator of collapsed motor mounts. They may be worth checking if you have not done so.

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1985 300SD, 203,000 miles, dark blue

Lil Sarah says: "Poor old Grandma Blue Car...She's sad."