I'm not a tech, but here is what I would look at if it were happening to me.
1. Flush coolant and replace with original mb coolant at the correct water to coolant ratio.
2. Check thermostat for proper operation.
3. check for bad hoses.
4. check radiator water filler cap and replace if necessary.
5. check for proper operation of the water pump
6. check that fan and fan clutch are operatng properly.
7. check for worn or slipping belt.
I wouldn't do it necessarily in the order I listed.
I would start with checking everything, then flushing, then checking more mechanical items like cap, thermostat, pump, etc.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.