Assuming all of the work you had done was performed correctly, I'd check the expansion tank cap. It should be a Mercedes specific 1.4 Bar cap. Usually, they will have "140" stamped on the top.

If it is bad or incorrect, it could be allowing the coolant to boil at a lower temperature than it should.

Also, check for air pockets in the cooling system and make sure the water pump and drive belt are ok.

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2000 c280 Sport (looks like an upgrade, feels like a downgrade)
1991 190e 2.3 (sold)