It happened to my '88 560sel. It happened once awhile at the beginning. Then in one week, it stalled progressively more frequently. I ended up have it towed because it wouldn't not go far enough before it stalled again.
Don't fool around with it. When engine stalled, you lost power steering and brake. It's very dangerous if you lose these while you are on a winding road.
Had the fuel pump relay replaced and it is running fine.