It doesn't sound to me like it has anything to do with ignition.
I would doubt that the Pertronix would fail intermittently. They either work perfectly or not at all.
You may be starved of fuel while under high demand. This can be caused by dirt in the carb.
Try to close the choke when it starts to stumble in third. This sometimes allows for fuel to be drawn into the intake, bypassing the dirt-clogged area.
If that doesn't stop the stumble, stop the engine immediately after it stalls and check to see if the carb bowl is low on fuel. If so, you either have a partially clogged fuel line, weak fuel pump, or the float is improperly adjusted (too low).
Carl
