Not Starting and running rich

My car is a 1987 BMW L6 (a 635CSi altered by BMW North America). It has 150K on its inline 6. 3.5L. About 4 months ago, the weather was very cold and the car cranked and cranked after work but would not catch. I cranked till the battery ran dead (because the starter switch stuck in the open position). When the temp got above 32degrees, I put some HEET in the gas tank and the car started right up. Then about a month ago, driving around 45mph, the car started running very rough, when I depressed the foot feed, the car cut out like it was flooded. Then, after a while, it returned to normal running. The next day, I drove a short distance and let the car sit for about 30 minutes. When I got back in it and tried to start it, it cranked and cranked but no start. I used HEET again. Started up. Last week, the car died at a stop light, sounding like it was running rich again. Finally restarted. Thought it might have been because of a bad fuel pump cause each time before, I was at around a 1/3 of a tank remaining. Took the car to a BMW dealer. They tested the pump (2.5bar regulated, 3.0 unregulated), scoped the gas tank, no sludge or water, they thought the mass air flow sensor but didn't really know. Yesterday, the car started and ran very rough again, sputtering, belching balck smoke. Got me to work, I couldn't floor it our it would cut out but would gradually raise speed to highway speed. Last night, didn't start again. Any help is appreciated.