Ehud: are a few things you need to do

1- take out all tail mixing from the radio.
2- make sure you have equal travel for your throttle from low to high ( ATV's set to 80% low and 80% high, this is just an example)
3- set your throttle curve to 0%, 25%, 50%, 75%, 100%
4- make sure you have equal travel in the tail servo from left to right in ATV's
5- leading edge of the top tail blades when vertical should be facing the rear of the heli and the tail rotor turns CCW
6- mechanically max out the servo travel so that the pitch arm hits the tail rotor and the tail gear case.
7- set your gyro to heading hold
8- keep your gyro gain under 37% in the gyro
9- pitch curve set to low -2 degrees, hovering +5 degrees, top end +9 degrees
10- if you can check the main rotor rpm's during hovering adjust your high end needle on the motor to get 1600 rpm's

if none of this helps you may have a problem with the tail drive, it maybe slipping.



Good luck

Alan......