This motor did the same thing about two years ago and it ended up being the module under the rotor in the distributor was corroded. I think I'll try replacing that again.

Intake/Vacuum leak??

The coil could be going bad.

Did you check the fuel pressure under load in the fault situation?

I had the same problem a couple years ago in a carbed HP500, the backfiring happened only under load coming on plane or when pushing the sticks, my problem was a b lown power valve in the carb. Replaced it and the backfiring went away.