The capacitors are getting old in the OEM CDI boxes and the output voltage of some CDI boxes is not what it should be. Like mentioned in an earlier post the BR (resistor) series of plugs reduce the "RF" (interference) but your voltage to the spark plug is reduced. Gapping the spark plug at .018" to .020" will often help some of these old ignitions fire the spark plugs when the voltage sent to them is sub standard.
Resistors spark plugs and or a 5k to 10k resistor spark plug cap is often required when using digital ignitions. If the ECU on a digital ignition experiences electrical interference, it will often cause a misfire or the ignition timing is altered on the engine revolution the ECE experiences the interference.