ash deposits caused by oil and/or fuel additives...if excessive deposits accumulate over a short time try changing gasoline brands. also do you add fuel additives and how often?this can also be caused by oil entering combustion chamber usually from intake side valve guide seals.
i did add some fuel stabalizer/water remover stuff this tank. i dont think it was oil. car was bucking like a bronco but new plugs helped a ton, just need to flush the rest of that crap additive out now i suppose
sounds good to me, i would run name brand gas only texaco,shell,chevron,citgo,ect.... stay away from those cheap stations don't know what they mix in there gas.. and recheck after a few tanks. do you always use the same gas station?
somehow i was thinking you must have just took it out of storage its probably a mixture of old gas and additives which fouled the plugs same as you stated earlier....