Were you using resistor plug caps with the (resistor) Iridium plugs ?

OEM plug caps are typically 5 K Ohm, same as the built-in resistors in Iridium plugs. I have had the resistors in plug caps fail open circuit more than once - try a resistance check between cable connection & plug connection on the (loose) cap. NGK caps part number includes resistance value (i.e LB05F = 5 K Ohm).

Mark Dicker had misfire problems on his R75 outfit (Boyer ignition with OEM coils). He ended up fitting a Vape self generating CDI setup (well he paid for it & I fitted it) as, despite sending scope waveforms & then the unit to Boyer (who found no faults) he didn't trust it. While working on fitting the replacement system (which included a replacement double-ended coil), I noticed that he was running resistor plugs with resistor plug caps. Too far down the line at that point to just change the plugs or caps for non-resistor types to see if that cured the issue - no misfire problems with the Vape kit IIRC.

That said, I have been running a Denso Iridium plug (resistor fitted) in the RE Classic 500 using the OE reistor plug cap for a couple of thousand miles without issues, so who knows if doubling the HT resistance is a problem ?

Nigel B.