Backfire through exhaust on long trips.

[charger440]

Hello, i have a 72 charger with a 75 440Ci + 727 trans.. I have recently had a problem where the car would backfire after returning to red lights and city driving from a 30+ minute highway drive. The engine will pause and then backfire when driving at a constant speed or slowly accelerating, however if i am hard on the throttle it seems to avoid the backfire. Driving around in the city alone and on shorter trips the car runs awesome.
      The cap, rotor, spark plugs and spark plug wires are about 6 months old, it has a new edelbrock 800 cfm carb on it, and the engine is stock with the exception of headers. I was hoping for some input and possible solutions from you guys here at this site since I have noticed how smart you people are from all the threads ive read 
I'm hoping its a sticky valve myself since i just picked up a new set of 906 ported heads.
  If u need any more info just ask.. thx in advance!

[Chryco Psycho]

sounds to me like it is rich or hot at idle , I wouldstart tunign it in better