I would also consider possible fuel percolating in the carburetor when flow is reduced after a heat producing drive.
What can you do to help keep the fuel cool before the carburetor and isolate the carb bowl from manifold heat?
Tie that heat valve open in hot weather--can you test this possible boiling problem by cooling the carb bowl when it performs at it's worst?--cool with a water spray in a spray bottle--maybe refrigerant.
Our modern fuel has so introduced some bad habits due to the lowered boiling point.
