I have battled with this exact problem for several months. It generally occurs just after the engine hits operating temperature.
The advice that I have gotten has been that the ECU is telling the idle valve to shut down because of bogus inputs. The usual suspects are the air flow meter, throttle position sensor, and decel micro switch. Vacuum leaks are also suspects.
Problems with the valve itself seem to be idle speed high or low or surging.
The only clue that I have gotten was when it failed when I had a meter connected to the EHA test harness. The EHA current went to +2 - +4 mA when it happened as opposed to swinging around 0 at very low levels when it was running well.
I can tell you what I have done that has not fixed the problem:
- plug wires have 15K miles on them
- checked for vacuum leaks
- cleaned rotor and cap
- new Bosch HD9C plugs
- new OVP relay
- new air flow meter
- new O2 sensor and lambda adjustment
- new coil
- checked decel and throttle switches
All of the replacements have made the car run better, the air flow meter especially. I put the coil in last night, and now when it goes rough it does not die.
I am going to do a post on this with more detail on the latest developments and see what the experts say.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe