Over Voltage Protection relay is one, cracked joints inside the relay is another, I'm going through the same grief with a 500sel at the moment. Idle Control Valve can be faulty, vacume leaks from the rats nest of hoses that live in the engine bay. Check also the rubber hose connectors on your I.C.Valve. These are fine on a cold engine, but once warm, they swell and you loose vacume. It probably runs fine, just when your slowing for a corner or stop sign/red light it stalls, but starts again immediately. Fuel pump relay is another gremlin. Check the cheap easy things first, Plugs and leads, distributor cap, coil before throwing big dollars at parts in an effort to find the problem. There are a heap of posts on idle problems, most point to vacume leaks or I.C. Valve, O.V.P Relay. The Idle control valve can be removed and the gunk cleaned out with Carb cleaner, a few success stories. Another is to disconnect the two wires to the I.C.V. start engine, if revs don't jump to 1500-2000rpm its probably deceased. Im no wizz on your problem and mine is still ongoing, I wish you luck, check the other posts, lots of knowledgeable people here.
Brian