TREX 450S Tail gyration - NEED HELP
 

First hovers with new Trex450S. Flys just fine for about 30 sec then the tail gyrates - moves sharply to the left about 6 inches then back to original position - repeatedly at a steady rate. It is not tail wag but a definite gyration. The same movement repeated at a steady rate. I switch the gyro to rate mode and this doesn't happen. Switch to heading hold and 30 seconds later it starts again. Motor increases and decreases RPM with each gyration.

New to Trex's and electric heli problems.

Align ESC, older Futaba GY240 gyro, JR 3400G tailrotor servo (new)

What do I check?? Help.