fx's
I belong to a club that races quickee 500's with OS 46fx's and 15% omega fuel (pink stuff). we will have 16 to 20 people on a contest day and I've heard of nobody going through that many plugs, ever. We use them because of their consistency from engine to engine, which makes for great racing.
The only thing I can think of is either too much prop or too lean a mixture. My opinion on prop sizes would be 9x7, 10x6, 11x5 etc. Don't load it down with a 11x7, 12x6. Let it spin, it's a 2 stroke. Good luck