Thanks to all who replied to my post regarding rough idle in my 1991 420 SEL. After the local Benz dealer AND a very competent shop went thru EVERY possible cause (so they said), and still not fixed, I went ahead and replaced -- guess what -- the IGNITION WIRES -- (hello?) they were the originals and were nearly 13 yrs. old! ! ! And the problem was solved. Runs great! Aren't I a genius? (No, the eye of the owner fattens the horse, thank you....)