Either the gyro is reversed or the servo is reversed. If you are holding full control one direction and it will hold somewhat steady, I would guess that the gyro is reversed. If you check and be sure that everything is like I said in my previous post, about the only thing left is the gyro being reversed. If you have built the heli and mounted all the gear per the instructions, the gyro should be reversed and the rudder setting in the tx should be set to normal(not reversed). Also, neutral on the tail rotor control arm should be square to the boom.
basmntdweller