popping through exhaust

I have been searching through back postings about the backfiring through the exhaust and have tried some of the fixes but still backfiring. 350 sb, new 600 holley carb, new wires, plugs, cap and rotor. The problem started before winter. I put on a new set of headers but had to take the camino to a repair shop for the exhaust pipes. When I went to pickup the car they said for emissons that they needed to make some adjustments. When I started it I noticed that there was a tapping noise from the pass side valve cover. Driving it home it started to backfire through the exhaust. Went back and they said that the tap was the problem. Talked to another person and their answer was the power valve was bad in the carb. Since then I put on a new carb and the other parts listed above. I have to check under the valve cover yet but after installing the new carb the idle was real low but it did not backfire nearly as much which makes me wonder if the problem is under the valve cover or something else. When driving at say a steady 40 the car jumps or bucks or launches something like that but it is not smooth. Sorry so long but wanted to explain as much as possible. Thanks