Hi- I have a 1990 420 SEl with the same problem. This is what I've done so far and it seems to have helped trememdously. First i replaced the ignition, distrib cap, plugs and wires. It got better but the best thing I did was put BG fuel system cleaner in it- I ran it through two tanks of gas and boy has it helped. PM "Johnhef" and he can explain it further, he's the one who put the magic elixer in the tank.

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"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL