I need help with my 2000 Lumina HELP!!!

My 2000 Lumina has approx. 47,000 miles and I never had a single problem until 2 weeks ago and it's become a nightmare ever since. Here's the atory..It was a hot day, we were attending a funeral. My Mom stayed in the car with the air running due to her heart condition for about 20 min. Then the air conditioner started to blow hot air and about 5 min later the car stopped running. We couldn't get it started, had it towed to a Chevy Service Dept. The car started for them. They tried to duplicate the 'events that occurred that day', they checked the electrical systm, left it running with the air at the max. level, test driving it shutting it off, starting it again, checked the battery, the alternator, the electrical system, couldn't find anything wrong. They had done this for 2 days. I picked the car up, drove it to work and back, it ran perfectly. I put the air about 5 min. before I turned the car off. 10 min. later I tried to start the car and it wouldn't start. It sounded like it was ready to, but it kept sputerring then stalled. I had it towed again to the Chevy Service Dept. after waiting an hour for the tow truck. It wouldn't start for the tow truck driver. We arrived 5 min later at the Service Dept. and of course it started for the service man. They had my car for 3 days, gave it a full diagnostic and could not repeat the problem. They said it could be electrical or the fuel pump. I called GM, they told me the same thing. It's very frustrating. I need the car for work, I have to drive through a dangerous part of the city to get to work and I'm afraid to drive the car now, I don't trust it. I really can't afford to buy a new car. I would love to know if anyone else has had a problem similar to this and how or if they ever resolved it. Help!!