Today I started up my 89 M3 no problems. Then about 2 miles from my house it quit running while waiting for a stoplight. It started right up it then would run for 20 seconds then quit. I would coast as far as I could then start it up and do it all over again all the way home. Once I made it home I idled it to see if it would stop,it did after about 30 sec. When it was running there was no miss firing until it acted up each time. Any ideas?