2001 Chevrolet Monte Carlo /Chevy//SS

Hey, I just recently bought a 2001 Chevy Monte Carlo SS. I found the car on craigslist, the mechanic was still working on it and I didn’t give him the time to finish what he started because I was in a hurry to buy the car. The car was working fine at the moment when I bought it. I drove around the first day and it was fine. In the morning I went to turn it on and it wouldn’t turn over, all of the gages came on and check engine light along with security and traction system came as well. I googled ways to fix this I tried leaving the key on the on position for a few minutes and tried taking of the battery to rest the computer. Taking the battery off seemed to work. My car turns on when it wants to, and when it does turn on it shuts back of with in a couple of seconds and or unless I keep my foot on the gas but once I let go it shuts off. I kept smelling strong gas smells and realized the fuel line was leaking gas and thought that was the problem, so I fixed it. But still my car wont start. I had a friend of mine come and scan it and it said it was MASS OVERFLOW or Volume. I later called the mechanic who originally worked on my car and he told me the reason my car was on and running the day I bought it was because he turned off the check engine light and rest the computer with his scanner. The person who sold me the car told me that he bought a new Mass Over Flow and put it on and it wasn’t the problem. During the second day of having the car and having left the battery of for an hour or so the car turned on. I drove around the corner for a little bit just incase and the car shut down. Now I Know that if the car shuts down it wouldn't’t be the security system that causes that. Was the reason it shut down was because it was leaking gas? I’ve fixed the gas leak already and the car still wont start, I think it has to be something electronic because the car turns on when the battery is unplugged and plugged back in. But know it doesn’t start at all it starts when it wants to and then shuts off. I’m going to tow it to the mechanic soon. I want to know if the NEWROCKIES system is what i’m looking for. Sorry I know I wrote a lot but i’m trying to get to the bottom of this. Thank you for your time