Been there, done that ...
The trouble with this sort of problem is that it could be one of about ten things wrong, and maybe even a combination. And without some serious diagnostic tools, you are kind of stuck throwing parts at it.
Given your symptoms, if it were my car, here is what I would suspect:
- vacuum leak at injector seal or manifold causing lean misfire
- bad injector(s) leaking or not opening at idle causing either rich or lean misfire.
So, I would see if Auntie would pop for a new set of injectors and seals - about $220. A very enjoyable wrench for you. Even if this does not fix the problem, it will eliminate a likely cause, probably make the car run better, and the injectors and seals will be good for another 100K.
You can do the injectors one at a time, and while you have one out, run the pump to purge the line and get any crap out of it.
After that, if it still misses, I would start looking into the electronics.
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