Traditional helicopter tail wobbles and doesn't hold its position

Hi, i m noob when it comes to helicopter i m running arducopter on traditional helicopter and using latest stable firmware for this but i m facing some problems with this thing the problem is the tails doesn’t wants to hold is position unless i try ro shake it left to right then it starts reacting but also after shaking left right the tail just wobbles to left and right non stop untill i lower the head spead significantly then it doesn’t happen , do i have to give some postivw pitch on tail rotor to jold its position , also i haven’t flown this thing yet all the test had been done on bench cause i m waiting for parts to arrive so i thought why dont invest some time in arducopter but now i m facing this problem i tried yaw tunning explained in arducopter page but that doesn’t seems to help i have tried everything i can think of , pls help