I had a problem similar to this. It was caused by the air cleaner being too close to the carb intake.
As I tuned the ped for more speed, when the rpms passed a certain point, I would get "blowback" from the carb, no doubt because my porting, exhaust etc. were not designed for higher rpm. This would soak the air cleaner in fuel/oil causing four cycling which slowed the ped down, stopped the blowback and allowed the intake air to dry the air cleaner, which allowed me to go faster....
Given that this started all by its self without you changing anything, I would make sure the air cleaner is very clean and that the carb needle and float are clean and working well.