My 81 300SD does the same thing. Sometimes I will be idling at a stoplight and almost rocking back and forth and magically it will settle down to a smooth idle - almost like a computer took over the engine management (but it's an '81). I have tried to remember whether it idles better in cold weather or warm weather but have not figured it out.

It absolutely seems like something starts working and the rough idle just goes away. 257k miles and I am the first owner.