A few thoughts about this one after consulting with a mate who has considerable experience in these matters.
The symptoms you describe can sometimes occur when there is a compression problem which may not be readily apparent just from a compression test.
If then bore is scored, then gas can get past the rings and collect in the crankcase leading to a backfire - sometimes the same thing can happen if the rings are 'stuck' with carbon or other residues.
As an aid to diagnosis, run the motor until it begins to backfire - then squirt a little 2-stroke oil into the carburettor. This'll help to seal any irregularities, and in the case of stuck rings maybe help to unstick them. If this appears to cure the problem temporarily (a few seconds only of course) you'll have to part the block from the crankcase and have a good look. Any scoring can be honed out, and the rings freed up, or even better replaced.
This advice comes to you from Frank Arnold - 40 odd years at British Seagull, and latterly the man who ran the repairs and refurbishment side of the company. He still repairs motors, and I ran the problem past him this morning.
Good luck
backfireing centry
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