I had exactly the same experience with my 83 380 sel. It was running perfectly, then I stopped at a light and when I gave it the gas it didn't want to go. Much reduced power, and bad misfiring. I was about 30 miles from home and the longer I drove it the worst it got. By the time I got down to the last 9 miles it was barely running, and the more gas I gave it the worst it ran. I was on the highway and couldn't get it up to 40 mph. The last clue was a strong smell of fuel.

I took it to my indie. he took one look and one smell said, "ignition coil." He had a good used one sitting on the bench, popped it on and the car ran perfectly. I bought a new coil (original Bosch - $65) and for good measure a new ignition control module (aftermarket Huco from Germany - $65) and haven't had any more problems.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire