1996 Chevy Camaro

I have a 1996 Chevy camaro that had a little problem when I bought it. The RPM used to drop down by 500-800 rounds while i'm driving. Then the car starts to shake as if the engine was dying and when I hit the gas the car would change gears and be fine. I took it to a mechanic who told me that I have to change the body throttle along with the mass air flow sensor. Since it was expensive new I went with the used parts. After he was done installing he asked me to start the car. When I did I heard a loud bang and the car didnt startm then I tried again and it did. When we went for a test drive, the car felt horrible: it wouldnt accelerate like before and the engine was loud. The transmission wont change gears unless I lift my foot off the gas. I took his parts off and put mine back, but the car stayed the same. I realized that he didnt know what he was doing and took it to someone else who ran diagnostics and found nothing. He changed the transmission fluid along with the transmission oil filter. Still the car does not accelerate and feels as if the engine is choking and it's very loud and hardley gets to 4000 rpm. I've changed the plugs and wires and that didnt do anything. Can anyone help me out and tell me what's wrong with it?