Likely the AAV (aux air valve) sticking. You could try taking it off and soaking it in solvent an/or simple green. Cool the bulb then heat it and repeat cycle including cleaning in-between.

The valve should close at about 150F and be fully open when cold.

If this does not work, you can dismantle the valve and give it a more thorough cleaning.

Several threads here in and in Benzworld 107 forum. This is a long one but you will find out how to dismantle fairly early on: https://www.benzworld.org/threads/aav.1592399/

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5