I think a wire or two got pinched and they're connecting and shorting the batter. I would have the gun rewired and make sure the wires in the grip are routed correctly. This is important because of how its seats the motor. Make sure both of the wires go to the "rear" of the grip, towards the stock, and route the positive wire "over" the motor to the positive terminal. If you need more help or pictures I could upload some.