2002 Chevy Camaro backfires

2002 Chevy Camaro 38,000 Miles. Hey I really love your site. This has stumped other mechanics. You're my only hope. My Camaro has a rebuilt engine but all the electrical components and sensors are the original. The car usually idles fine, but when you put it in gear and step on the gas it sounds like it backfires in the engine up by the air filters? This happens on Initial acceleration but gets better like on the freeway. But a pain on the streets. Code 21 and 22 come up (TPS). I changed it and it was better but a couple months later it slowly came back. I took it to a mechanic who changed the TPS again and said it was fine now. Months later the same thing happened, it came back. I went to another mechanic and did it all over again. What could it be? Could it be the actual computer that's fried? If I go to another mechanic, how do I know they know what they are doing because they seem to know about as much as I do with this problem? I love my Camaro even got it a new engine that I have never even really gotten to appreciate yet. Please any suggestions or insights would be great.