Sounds to me like the electronic speed control's low voltage cutout is kicking in. In order to prevent over-discharging lipo batterys, ESC's have a circuit that warns you of a low battery by shutting the motor off. Going to low power resets the LVC so you can still land under power.

Make sure the battery is fully charged. A low battery will trip the LVC at full throttle, that's the LVC doing its job, not a fault.