The engine is getting too hot. I would back off on the nitro to 5% for starters. Not knowing what your install looks like as far as airflow through the cowl is concerned makes it difficult to offer any other suggestions. Prop wash does near nothing for coolin in a cowled installation. Cooling is a result of keeping air moving through the cowl while the airplane is moving forward. As a rule of thumb you want the exit opening to be at least twice the area as the air intake.