I spent 18 months hunting down a surge and stall problem like this. I changed plugs, distributor cap, air slide valve, idle speed control, OVP, fuel pump relay and finally found that it was the ignition control module. Look in the driver’s side fender well and see if your control module is loose. Also, see if it really hot, not just warm. Mine would get scalding hot. If you have a problem identifying it, it is a silver box that has a lead to the coil. Mine was loose and so was the one in my friend’s car. His was still good, but mine was bad. Replacement (Partsshop) cleared up the major problems. Look for unstable tachometer readings when it is going nutty because they share a ground. My fuel pump relay also was clicking like mad. Good luck because these are not easily diagnosed.

Steve
’85 500SL Euro