I have a 1977 280SE and lately mine's idling rough too. When I put it on gear, it hesitates for a few moments and then it runs smooth after that. This same incident happened to me a few months ago. I just added a canful of carb/fuel injector cleaner and switched to Shell premium gas and just after a tankful, my car's been idling smooth. So I guess it's time to add some more juice or what?