I've been having the traditional rough idle problems with my '84 380SE. Tried a bunch of things, but to no avail. So, I thought this was one of those things I have to learn to live with.
Well ... I was snooping around under the hood this afternoon, and I pushed down on the wires to the distributor. And guess what. I could push two of the connectors down a little more. Felt a bit of a "click, click" as I pushed.
I jumped into the driver's seat and turned the key. She started right up and idled smooth as can be.
So, for those of you who have a rough idle, check to make sure the plug wires are firmly attached to the distributor. In fact, double-check.