hi, i'm not claiming to be an expert by far, however i had similar unhappy hot car issues when i moved from NY to California (suprisingly before then i never had problems in 100 degree NY weather) i think high heat + humidity is certainly better than dry heat.
anyway - i went to walmart and found a "special" race car coolant that claims to lower your running temp 20 degrees. i bought it and put it in my radiator.
it keeps my engine at a steady 80. again, not claiming to be an expert, just throwing it out there. good luck!

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1985 300D - "Desi"
1981 300D - "Pepi"
1984 240D - "Sticky"
1985 300TD - "Flitstone"
1984 300TD - "Frenchie"
1987 300TD - "4481"