How long have you had the car? Any history. Low miles on a old car means its been sitting a lot. I would bet the head gasket is all you need. Without any prior history it is tough to tell.
You could pull off the cylinder head and see if you find the problem. There may be some tests you can do to see where the issue is without disassembly but i'm not for sure. You can search it on google, the issue is not Benz specific. I think if there are any tests available they are universal to all makes

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Chris
84 280sl
82 300d euro