hmmm, i was also thinking it was a gyro issue (it may still be a bad gyro but we'll check some other things). other than that it may be the tail servo is not centered.

check the position of the slider on the tail shaft and see if you have full range of motion when moving the rudder stick back and forth on the tx. the tail servo should be positioned such that the arm on the servo is at 90 deg and the slider is at the mid point on the tail shaft.