92 Lumina euro stalling and starting problems

I bought a Chevy Lumina Euro 3.1 about 2 months ago and it just recently began giving me problems with starting and stalling at stop lights and signs. At first I thought it was the battery because this didnt start happening until I let a friend borrow the car. He forgot to turn off the lights overnight so the next morning the battery was dead. Well I replaced it with an identical brand new one, but about 2 days later it began dying everywhere, which it has never done, and it also had issues with starting as well. I would try to start it, and it would start for a sec but when I tried to put it in gear it would just die. Then it started to just die right when it started up without me even trying to put it in gear. Sometimes it would start up and run, but that seemed like a random occurance. But I have no idea what is causing this because I brought it to the mechanic and they couldnt find out the problem, some mechanics, but I was just wondering if anybody out there knows how to fix this?