I agree with Jim, sounds like a bad head gasket and head warpage. When you say "overheat with no water leak", how low is the coolant in the tank? A bad head gasket could be dumping coolant into the combustion chamber, losing coolant with no external sign. Check your expansion tank for telltale drops of oil floating on the coolant.
How long did you run it hot and what kind of shop did you visit? Doesn't sound like a MB shop if they turned it out in the condidtion you described.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
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