david-having similar experience with my '86 560SEL. What i am finding out is that the engine mgt system on this vintage of car is amazingly complex and subject to component failures that are hard to diagnose. In our model year online diags were/are not avialable as I understand it. I have not been able to solve my hard start , cold rough idle problem but patience is key I guess. A few thoughts on your situation ( I am no expert) -
-have u been through routine "tune up"?? The high mile cars need a certain amount of ignition mtc - esp plugs, rotor,cap etc. I am not a big wire replacement fan (replace em when they fail) but that could be done too.
-depending on mileage (greater than 100K miles) new injectors might be another thought - preventive as well as addressing your problem .
-check the coolant temp sensor for resistance and wiring continuity.
-I am starting to lean toward fuel distribution on my car (165K miles) but want to look further. It is a very expensive replacement. Also , check and replace fuel filter near the fuel pump and inspect what junk comes out of the old one.
My $.02. GL and keep us posted.