every once in a while when I fly my cx2, I get it flying in the air and all of a sudden the red light on the 4 in 1 will go on and the heli will drop to the ground. as soon as it lands on the ground I power down the throttle and power it back up and it flies fine again. I know it cant be the battery is low because i never fly on a single battery for more than about 10 minutes. This drop thing happens as soon as a couple of minutes into the flight. Does anyone have suggestions of what is happening?

I have a heat sink and i dont run but one battery at a time, i let the motors cool back down before i fly with the next fully charged battery.