I experienced a similar overheating issue with my 1985 300D at round 260,000 miles. I found a significant amount of debris trapped between the condenser and the radiator that effectively blocked about 30% of the surface area. In addition, a large number of the radiator fins were blocked with bugs, dirt and straw. I pulled the radiator and power washed it and also power washed the condenser coil in place. After reassembly, my average temperatures dropped from over 100 C to around 85 C.

I subsequently flushed the cooling system, installed a new MB thermostat and fresh MB coolant. Since that additional work, the average temps are now slightly above 80 C with ambient temperatures above 90 F.

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Blair Tyson
1985 300D
300,000 + miles