My 83 380 sel has had these symptoms for a while when the car is on Neutral , Park and stopped in traffic (Drive). You sometimes can see and feel the car shake. In stop and go traffic, I sometimes feel the car hesitate, but then drives fine after a few secs, in highway traffic, car drives like a charm.The firing is also up, down, up down, up down, (in other words it doesn't sound like it is firing right.) The car has had the fuel pump changed, by a the Merc Dealer. I just last week changed the Cap and Rotor, and the Ign Wires. I had changed the plugs about 20000 miles ago to Platinum 4's ( which I am kind of not sure about now after reading a few posts). yerterday I noticed, a really small rattling noise , which I believe me may be linked. And to top it off I failed emissions when I took for the 2 year New Jersey Inspection. Somebody!! Anybody help!! I love this car too much, it has the nice one piece Euro lights, the shorter bumpers, you know it is a Euro. Oh Ya and it has no oxygen sensor, I checked that already.