I have an 82 300sd that had the almost exact same symptoms. Its deffinetley the vacuum system. My problem was fixed almost immediatley after i discovered a hose that was disconected right by the vacuum pump itself. Looking at the engine from the front, the vacuum pump is located to the right rear under the hood (right above the front left tire and right next to the main fuse box). Just follow all the lines from the pump and see if you can find any leaks or disconnected hoses. This of course is assuming your vacuum pump works. Anyone with better knowledge please correct me if I am wrong.