Denny touches on it and I have the same inkling.. when cold, it runs well, when temp comes up it backfires.. I'd say check the valve adjustment. It "feels" like the valves are being kept open when the system gets hot.

Also, the Accel dizzy, I wouldn't trust any part of it. They've earned their reputation.. it ain't a good one.
Once upon a time, back in the 60's, you couldn't win without them.. but now?? Don't trust - verify!