The OVP relay supplies power to the ECU as well as the ABS. So a flakey ABS can shut the ECU down which shuts the idle valve. That is why an ABS light was a what I thought was a major clue.
The night that it happened, Senior Management and I were on our way to dinner. The really bad stalling episode happened and we sort of rolled into the parking place. After dinner I started the car and it went into sort of "limp home" mode, with a high idle but running OK.
If you are getting an intermittent ABS light, I would definitely replace or re-solder the OVP relay.
About all you can do with this problem is test the wiring, and then throw parts at it because there are really very few diagnostic tests.
This is not so unreasonable as it sounds. All of our cars are old with lots of miles. All of the components have been hot, cold, and vibrating, for at least 15 years. If you need to replace the air flow meter, then the EHA cannot be in very good shape and will probably be the next to go. Same for the idle control valve. Same for the OVP. And the injectors have been subject to constant abuse.
Thee is no magic bullet - if we want good-running high-mileage 103's that will stay that way for a while, we are probably going to have to suck it up and replace all the stuff that is at or near the end of its useful life.
FWIW, when Herr Fuchs gets a car with this problem, his advice is to check the coil and replace the following:
- decel micro switch
- throttle position sensor
- air flow sensor
- coolant sensor
- OVP relay
- all vacuum fittings
- idle air hoses
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