good luck too you, these are the worst problems!!
sounds like something heating up after time, could be piston growing and semi seizing (insufficient clearance) . on a scoot could be electric choke not cutting off and over fuelling, could be tappet gap closing up, could be cdi overheating.
finally on a fuelling point, low compression ,lack of vacuum or hole / cracking of vacuum pipe not operating fuel tap diaphragm.
cross out any of the above that do not apply, ie 2 or 4 stroke valves. manual or vacuum op tap etc good luck
