You mean rotating on it's axis ? - I.E the tail is going round in circles - or rotating as if the pitch is out of trim correction?

As everyone else says:
Check the inner shaft is straight,the retainer collar is still tight and the gearing isn't slipping. Check that the blade retainers aren't damaged. Check the flybar is properly balanced, and located accuratley.

It the tail is rotating like mad, then make sure your battery is totally charged, both servos are mechanically centered, the push rods aren't bent and the swash is level and in one piece, and also that the retaining finger on the swash is located in the guide rail.

Set all your trims to center, and turn the TX on, then V4. Let is settle for a few mins until the status light is green, and apply throttle. if the tail starts rotating immediatley, try and correct it with the trim, if this doens't work, then go through thte normal routine as per initial setup.