It sounds like the vacuum modulator. Just like your 240D has one. There is a vacuum line that runs off and down to your tranny. Unplug it and put a vac pump on it and see if it holds vac. I if not then there is the problem. It's a 45 dollar repair. Just when you take it out, whatch out for the plunger inside. You brake that and good luck getting it out. The Punger part is cheap, just not easy to get out if broken inside.
Later,
Rob