Is it the standard motor or is it an upgraded one?
Is the battery pack fully charged?
Sounds like the problem is essentially the suppply of power to the motor.
Either the batteries can't supply as much as the motor is demanding (faulty pack/low charge), or the motor is trying to demand too much (overpowered motor/mechanical fault causing excess strain).
Also possible are a faulty speed controller, bad contacts, or a badly suppressed motor (solder on capacitors, often pre-fitted on modern motors).