Have you noticed a loss of power associated with your overheating issue? If yes, then check the head gasket or the valve adjustment. My old 84 300SD began overheating and the same time I noticed a loss of power. All the cooling system components checked out OK. It turned out to be the head gasket was leaking and thevalves needed adjustment.
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!