Dose it ever happen cold, or only after warming up? I only ask because I wonder if the gas (modern) is boiling in the bowl and then when it sloshes forward the main jet is only sucking the foam that happens on top when fluids boil. If this was the case it would refire as soon as the liquid sloshes back enough to cover the main jet. A heat shield or spacer below the carb could help if that was the case.