Intermittent problems are the most difficult to find.
Check the fuel delivery, as suggested.
Also pay attention to where your spark advance is when you have trouble. Something could be grounding out as the point plate is being moved.
Also check the points/point plate to make sure you are getting a consistent gap every time on every cylinder. I've only seen it once, but recently I had one that had been converted to modern points that was shifting around, sometimes the points would open and sometimes not, effectively turning the ignition off and on and causing a backfire. And yes, it just started out of the blue one day.
Also I'd try bypassing the switch, just to eliminate it.
Good Luck to you!
