bounce

i have an fz 750cc & after fitting a new set of plugs approx 6 weeks later my No 2 & 3 cylinder plugs oil up resulting in 1 or both cylinders miss firing,even though i may be on a journey at the time i have still encountered a plug oiling up & not sparking.upon cleaning said plug, the cylinder will spark,as it should .this can happen suddenly.all sparkplugs are indicate a perfect mixture generally,but this is a reacurring problem with thisz bike.has anyone out ther had a similar problem, please help