Vacuum is your problem.

The system is equipped with reservoir(s) that allow vacuum to remain on the various flaps while you accelerate (low or no vacuum condition).

Your problem is a classic example of a vacuum leak in the system.

I would expect your car to be rather complex, so an accurate schematic of the vacuum system is required to troubleshoot. It can be tedious, but it is not rocket science, so it's doable by anyone with light duty or better skillset.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles