No help here, just adding that my 1991 560 SEL has the identical problem. In fact it's done this for several years...I've just learned to adapt because I, too, have taken it in (to three different shops) with no improvement, despite fuel injection cleaning, gas/air ratio adjustment and distributor cap replacement. Maybe it's due to one of the problems noted above. The idle control valve sounds reasonable. Maybe I'll take it back in...