I have a 1985 318i. With time, it got more difficult to start when warm, so I did some troubleshooting. I found it was the in-tank fuel pump, and replaced it. It ran better for a few days, now it's doing something new.
If can drive 4 miles away(It runs fine), and park it for about 45 minutes. When I'm driving home the car will die about 1/2 way home. After it dies, it's difficult to restart, but will after some persistence.
When driving on the highway, the car will also have power surges, however, it doesn't die.