I had simillar problems with my 1987 300D .... I swapped out the worn motor mounts which smoothed out the engine tremendously and I replaced all the worn vacuum hoses in the engine compartment and the idle problem went away. I'm not sure which hose I replaced solved the problem but the hoses on the passenger side that were connected to the EGR, Turbo, and the vacuum controls by the airbox were in really poor shape and obviously leaking so I suspect those contributed to the problem. I suggest starting with getting a couple meters of vacuum hose and a few molded angled fittings and just start replacing lines one at a time. Some lines may look ok but they may just crumble or break when you try to remove them. Good Luck!