flathead possible miss only when hot?

[hombres ruin]

so to continue on..i have got a new coil and a new ballast resistor one that is recommended by Mallory for my magnetc breakerless dizzy.the carbs have been rebuilt and spacers placed to stop heat sink...The flathead runs really nice until it gets to its op temp and a small flat spot or stumble if you will starts again..I can maintain speed with no stumble and it occurs on idle ,it never wants to stall at all.could this be ignition such as a breaking down distributor,or spark plugs..some people have said vavles..I don't know what else to do