I was having problems with my '88 260e stalling intermittently, mostly at idle or when coasting to a stop. The problem turned out to be the airflow sensor pot. It is a small black rectangle on the front of the airflow sensor with two wires that senses the airflow plate position. In time, the electrical trace wears out and causes a hiccup to the ecu that confuses it enough to kill the engine.

The dealer will tell you the entire airflow sensor must be replaced but the exact bosch pot can be purchased separately for about $80 from olyparts.com. Replacment is very touchy and I think it is covered pretty well in some previous posts.

I'm not saying for sure this is your problem but with the miles on the car and the symptoms, it's worth checking into.