[mark houghton]

Hmmm, weirdness. So, this is backfiring just sitting in the driveway idling?

Sounds like even with pulling plug wires to try to isolate it, no change occurs. So it aint wires or plugs, that would suggest. Did this really start when you changed out the rotor, or could your valve adjustment coincidentally have anything to do with it, since all was well prior (except for melting your resistor rotor). I wonder if your MSD or coil are failing you? How's the dizzy cap look inside...any evidence of carbon tracking? Also, might try hooking up your timing light to see if the timing is dancing around when it backfires, and then hooking up the light to each individual plug wire as close to the plug as you can get, to verify consistant spark to each cylinder.

Also, have you watched the engine bay in complete darkness to look for arching? You know, in the garage with the lights off and an oxygen bottle hooked to your face so you can survive the drill?