Headers help

Im at a loss lol I got my headers on my 03gt 6 speed and put on a ebay exhaust to easy right, well I started the car up and it races and alost stalls ans does it over and over also Im getting a nast popping sound Im guessing back firing. all the o2 sencers are in an tight and all the bolts are tights also I hade the intake off could it be a vacuume leak?

Thank you

Travis