Hrm... Could be a number of things. First check the pack, all the battery bars on it should be secure and undamaged. You said you checked the battery plug so that's all good. Do like travis said and go to the esc now, check where all the wires come out of the circuit board.. After that, go to the motor, check the connections there. If at all possible, try a different motor.
Oh and do you have a diode attached to the motor or no?