I had the same **** going on with mine. I built the 450 SA ARF kit and used the 401 gyro. it would hold fairly steady then the tail would start to wander and not hold. towards the middle of thebattery pack it got worse. I would correct the tail sway and 30 seconds later it was wagging like a dogs tail. I always noticed that when i connected the battery pack and the ESC beeped that the rudder servo would "twitch" with each beep. the swash servos didnt move. I finally figured out that the ESC was just too close to the gyro. I had the gyro mounted on the back bottom plate. I had the ESC pushed forward but it was still too close. I ended up moving the ESC forward almost directly under the motor and made sure the motor leads werent close the the gyro. BOOM!! problem solved. it holds like a rock and holds better in the hover.

Hope that helps.

it was the motor leads that did it, not any of the input power wires.