The ignition switch sounds like a likely culprit or the opinion of MBDoc.

Must be a freaky wire that periodically makes contact imitating start up. Your car's check engine light probably goes on moentarily upon start up and a few seconds later goes off. That could mean that the relay MBdoc mentioned is freaky. Energised when it shouldn't.

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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