that prop is way to big, if your getting 9300 on a 2 stroke you are well below the powerband, you should be in the 12000-15000 range. try a 14x6, 13x8 that will make all the difference in the world.
the os manual is for japan, which regulates noise from the engine, in the USA we dont really care (as long as it isnt crazy loud) so you want your rpm's much higher. your engine will make a lot more power in the 12-15000 range

whats most likely happening is your engine is overheating, or your way to rich cause your prop is way to big (im guessing you have the needle at least 4 turns out)