Could be the speed control is overheating and shutting off the motor.
Does it get hot? Does it have enough air flowing by it?
Something might be causing the amp draw to be higher than it used to be.
Could be the motor is going bad and drawing more amps.
Are you using the original prop? Could try a smaller prop.
If it's a geared motor is anything binding?
It could cut off if the battery voltage gets to low, so it could be the battery.
It also cuts off if the speed control overheats.