i have got an odd problem with a 50cc Air cooled Two Stroke  scooter engine that keeps locking up at full throttle then freeing up again. I just bought the bike second hand with a new belt needed fitting.
i have pre-mix oil/fuel in case the oil pump keeps failing.
it ticks over no problem and has plenty of pull. but when i full throttle for more than a minuet the pitch of the engine drops suddenly and the engine sizes and back wheel locks. I dont know why it is sizing up. 
I had the head off and checked the piston and barrel and rings. all look good. i really have no clue now why this is happening