This should help with the plug codes. Nutshell ir is a reistor type
http://www.ngksparkplugs.com/techinf...tnumberkey.pdf

FWIW Found this via : http://metal.duncanamps.com/links_info.php which is neat..

If you doubt the plug spend a few bucks and buy a new one.... heck of a lot less cost than your plane and time repairing it.

Take a real close look and verify you have no metal to metal contacts that may be vibrating. I may be over cautious be I choose not to use any metal clevises on control rods (if I did I'd probably CA them after adjustment. I had glictching while using a makeshift plane alum model stand (modifed walker), off the stand no glitching. A new wood stand is in the works.