If you're using the GWS 3in1 mixing board, that is most certainly the culprit. The PHA-01 board is just awful, it constantly has problems. The FETS for the ESC's are located right beneath the gyro on the board, and as they heat up, they also heat up the gyro, causing really bizzare tail control, random tail movement, and the tail even spinning while the main blades are completely stopped and the helicopter is staitionary on the ground. During flight, the system constantly requires trimming to compensate for the 3in1 as the gyro heats up. Eventually you run out of rudder trim and need to land to let it cool down before flying again.

If you're not using the 3in1 board, then your problem is rather bizzare, I would guess something to do with the ESC for the tail, possibly supplying to much, or not enough, voltage. Or maybe there's just a loose connection somewhere. I've seen wires (especially if they're cheap ones) fatigue off after extensive use, coming seperate from PCB's and the like. Maybe someone else will have some brighter ideas. I had gotten fed up with the GWS 3in1 board and replaced it with a cheaper Walkera 4in1 and have had no problems since.