Wants to stall

I have the LT model. I noticed that when I run my air conditioner on high my cars idling starts to drop and when I make a stop the car will stall and feels like it wants to shut down. so I run the air on level one. well this year I am not able to run it all and now it does it every so often when I make a stop and at times it will back fire on me and inside my car it will smell like gasoline. I have taken it to the shop they ran a diagnostic machine and all they can tell me that they see nothing wrong. I am confused why it will not show on the machine. Keeping mind it does not do this all the time. I have good days and my bad days are when it i super hot outside. Please help me and thank you. I thought maybe my fuel injection pump or something.