Hi everybody. I have a question for you

I have a Raptor 30 V2 with a RD6000 radio. The gyro is the Futaba GY401 and the rudder servo is a Hitec 525.

The problem is that when the Heli is in hover it has a slow yaw movement. The nose of the heli goes clockwise with small 'hits' of about a couple of degrees every 5 seconds or so.

The tail blades are slightly damaged because they hit the ground a couple of times, but I dont think this is the cause.

Any ideas?. Thanks a lot.