Also check the O2 sensors, my car did the same thing, I switched to LT headers so I replaced the O2 sensors and the problem went away. Run the codes first like dstanggt50 said.
Tim
Do you have any exhaust leaks? I stripped one of my header bolts so I left the nut off it. It's on there pretty good while i'm ordering a new bolt kit but once in a while my car will backfire on load and sometimes with no load because of the leak (which you can't even hear but it's there). I'd check for leaks too!