The aux. fan is going to be on if your a/c is on, especially in phoenix.

If you are sitting in traffic it's pretty hard to get the air to move across the radiator.

Try using Mercedes Benz coolant with distilled water. Is your water pump, thermostat, radiator, radiator cap in good shape?

Consider a new radiator, etc, or a custom one made, and possibly a new fan/fan clutch.

Take off your belly pans (the covers under the car) off, that will get some more air moving around, but consequently the engine compartment will get dirty.

How how was it today 108 degrees F outside, and you are running 110 degrees C on the water?

Hope this helps.

__________________
Current Stable:
1994 S500 v140, 210k miles, white with grey.

Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold (rumor has it, that it has 750k miles on it now)
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 "300E" Amg Hammer 88k mi sold (it was really a 200, not even an E (124.020)
1992 500E 156k mi sold
etc.