These models can be a bear to make idle smoothly. I'm a 30 year veteran MB tech and my own 380 SEC has quirks that I've learned to live with. It sounds to me like you may have a vacuum leak which can come from numerous items. Squeezing off all the connections to the intake one at a time with a pair of needle nose pliers may find your problem, but the plenum seals as well as the intake gaskets could be trouble also. Spraying carb cleaner on the suspected areas could show-up the leaks. Water poured over a vacuum leak will also be a good tool to detect trouble. If you isolate the weak cylinder to a single cylinder try swapping a fuel line from an adjacent cylinder. If the problem moves to the other cylinder then the problem is with the fuel distributor.