I had the identical stalling problem with my '88 260e. The culprit turned out to be the airflow sensor pot. It is a variable resistor on the front of the airflow sensor that senses throttle positon. The carbon on the circuit board wears out first in the idle range (where it rubs most of the time). A slight blip (loss of signal) is enough to kill the engine with no warning at all.

You may be told that the entire airflow sensor needs to be replaced but I bought the sensor pot at Olyparts.com and changed it myself. Hasn't stalled since.