Answer may be coolant temp sensor
Chances are that if your car is getting into the 90s - your thermostat is already at full open. In this case - a lower temp thermostat won't help. What will help is modifying the temp at which your auxiliary electric fans come on.
I'm not sure about an E320 - but on my 1995 E420 - there is a coolant temp. sensor that lives only to control the aux. fans. (a separate sensor gives temp signal to the engine computer). Jim F. has a website that shows how to modify this sensor to "fool" it into thinking that the coolant is hotter than it is - bringing the fans on at lower temperatures. Involves about $2.00 in parts - simply wiring a resistor in parallel with the sensor. You can choose your own resistance depending on how low you want the fans to turn on. I chose 1200 ohms (about 95 degrees) - but i believe that 1100 ohms will make it around 92 degrees.
Someone with an E320 whould be able to let you know if this is possible on your car. Anyone?
1995 E420 65k - trying to survive the dreaded evaporator failure