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!
1980 450SL @ 103,000 miles
1966 230SL @ Unknown Milage
2006 C230 @ 110,000 miles (Sold)
1994 E420 @ 118,000 miles (Sold)