pipe popping

You said it ain't the plugs, but we had the same problem years ago in a Challenger 340 car my brother owned. Did the same thing you're doing now...went through everything, still popped. Turned out to be a cracked insulator on one of the plugs causing erratic firing, and igniting the unburned gas in the exhaust system.

I agree with the poster who mentioned pulling one plug wire at a time, if it stops, I would concentrate on that cylinder, and replace the plug first!! Geof