Desperate to find out what's wrong with my car, Mini mechanic failed me.

I drove my car down to Maryland last summer from PA, on my way home I stopped to pay a toll and while I was stopped, my RPMS kept going up and down and my car was jerking back and forth. Then it stopped, when I went to hit the gas, nothing happened. My car had stalled even though it's an automatic. I has to turn my car off and back on again. I drove in fear the rest of the way home and it happened again, when I was slowing down to make a turn. It happened on and off. I brought my car to the mini dealership, the guy said it was my spark plugs and replaced them for $400 and told me all of my notification lights were off. When I picked my car up, a brake light and another light were still on. Tried to get in contact with him but he wouldn't return my calls and then found out he later on quit. I wasn't mad because my car stopped stalling out and shaking. About a month ago, it's started again. It happens when I slow down or brake. Today when I stopped at a red light my battery light came on and then it started jerking before it stalled on me. The battery light has never come on, I just bought a new battery last winter because my car wouldn't start when I would get out of work, I guess because of the cold. I bought my car is 2012 from a private dealership, the owner had lived in California and took excellent care of the car and drove it cross country. Please help, I can't be taken advantage of again being that's what j felt the dealership did about my spark plugs. My car has oil in it, I used to run my gas low before filling up but I make sure it's half full now. I'm desperate, please help!