sounds like early valve timing which can be caused by seats receding into head which acts exactly like over adjusted valves. most of the time this backfiring occurs when steering (which lugs the engine) and attempting to accelerate. occaisionally lowering the steering pressure will help. if the valves are opening early it should show up on a vacuum guage. (a really good diagnostic tool that everyone seems to have forgotten about). make certain even with new parts that moisture is not getting into the cap through tiny cracks. i have had this happen in the past. mist the cap with water and see if the problem gets much worse. can be a hard problem to find.....hope you get it knocked out soon