This may sound silly but make sure your plug wires sequence was correctly plugged into the distributor cap.

Also make sure you have the correct plugs.
Some types, although categorically appropriate for your w126, may not behave the best. Its been a while for me and a W126, so I can't recite the plug types.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD