For the idle, you may wish to check the throttle switch, at the base of the throttle. It tells the computer the car should be idling. (Switch is closed when the accelerator is not depressed, open any time the pedal is depressed).
The 1985 has a different idle control than my 1989, so I can't help much after this. You may wish to search for idle problems with the same year 380SEL. I think these are the same system, and I recall noting some posts on this model.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)