Roach
Also - I suggest checking the HT leads. A dead plug MAY be caused by a failing/faulty lead. Mine was. Just like you - I had 3 pots going, found one failed plug and replaced it - all went well for a couple of days. Became rough..in the end - I found #4 lead was hanging by a thread...such that when I pulled it off the plug - it came off at bothe ends. Rebult the coils to solder in new HT leads.
Regards
Mark