My '90 300SE had a slight miss at idle. I replaced plugs,air filter, no change. I found the coil wire touching the intake manifold and it had a heat mark on it. I took a wire tie and corrected this.
That seems to have taken care of the problem. I saw a post recently of someone asking the same question. I can't find that post, but maybe he will see this and give it a try.
84 300SD (sold)