With a car that complex, it could be many things. My "daily driver" is a much less expensive car, but it had numberous O2 sensors, both "upstream" and "downstream", go bad on it. Symptoms included:

Check Engine light on
Code regisered in OBD (On Board Diag)
Occasional fast idle
Occasional rough idle
Occasional stall from idle
99% normal operation while driving

I'm sure an O2 sensor would generate a code in an MB if it creates one in a cheap Dodge. I think you might need to look elsewhere if you don't have a code.

Since it happens only when the ambient temp is high, perhaps you need to look at air/fuel mixture sensors and systems. As the temp of air increases, it can carry more humidity and it gets less dense (less oxygen). I am not a mechanic by trade, I'm just trying to use my common sense.

Best of luck!

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