Sounds like a restriction in the fuel system to me.
Petcock screens in the tank, petcock, filter at the inlet of the carbs, are a few places where there could be debris restricting flow.
When it stops, you could open a floatbowl drain to see if only minimal fuel comes out. That would confirm a restiction.
The filter at the carb is inside the inlet, and is easy missed if you don't know it's there.
Inline filter -
https://faq.ninja250.org/wiki/File:Fuel_Filter_4.JPG