As you probably know, intermittent problems are the worst to troubleshoot.

Besides the OVP relay and ignition system, there are a couple more things that you can check out:

My '88 260e was hesitating very badly as the engine warmed up. The car started fine but as the temp gauge got to about 60 degrees C the car would stumble and almost die when accelerating. Once it reached normal operating temperature it ran fine again. As you said, it was dangerous to pull out into traffic. My problem turned out to the EHA valve on the back of the airflow sensor that enrichens the fuel mix at the car warms up. The part is easy to replace but not cheap (about $200). I replaced mine mainly because it had a slight fuel leak and after replacing it the hesitation problem was fixed.

The second possibility is the airflow sensor pot on the front of the throttle body. It is an electrical potentiometer attached to the throttle plate shaft that senses the throttle position. The pot wears out first at the idle position and an intermittent circuit failure can cause the engine to die. My car would die occasionally with no warning while idling or when coasting around a corner. Red lights would just appear on the dash. The problem got worse over the course of a couple months until I was afraid to drive the car.

Do a search on "airflow sensor pot" on this forum for more info. The dealer will want to install a new airflow sensor for big $ but I bought the pot separately for about $80 online from olyparts.com. It is a VERY sensitive part to replace and you must be careful to scribe alignment marks on the housing to get it back in the right position. I have heard of people having problems with corrosion on the pot that they successfully fixed by cleaning but mine was worn and not dirty and had to be replaced.

Good Luck!