Sounds like it is getting very hot and you are getting vapor lock.
Is it baffled? I'd also suggest that is too much prop for the engine and you are overloading it.
Advice I got for mine from the guru Ray English was to use a 20x12. That is in a 3 blade prop.
2 blade possibly 22x12.
Consider going to 15:1 klotz as the additional oil will aid with cooling.