Make sure your head screws are in the shaft holes and the lower gear is positioned with the set screw on the flat part of the shaft and is aslo tight. I'll bet the lower gear set screw is coming loose or you are not lining it up with the flat spots on the shaft, so when you fly the gear slips and the up rotor dosn't spin as fast as it should and the lower blade torques the heli around to the left.[8D]