I think you hit the nail on the head. Revving the engine would crank up the vacuum pump overcoming a leak threshold that prevents the vent door from changing position. I'd venture that once a certain RPM is passed, whether under hard or mild acceleration, that the door would change positions and stay there until the reverse threshold is passed.

It could be as simple as a loose connection on a changeover valve, or a pinhole leak in a pod diaphragm. You won't be able to tell until you isolate down to the vent and break into the vacuum system.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)