As you said in the vid, I too wouldn't expect the slight non-centering of the servo arm or slight off-zero on the blades at reference position to be affecting it this badly. Ideas:
1) It shouldn't matter, but maybe it is: try moving the links (or the entire tail servo bracket) so that the blades are zero or even slightly angled to give tail left/nose-right thrust to counter main rotor torque. As it is now there is slight thrust tail-right which is in the direction the torque wants to take it. It's possible it is confusing the gyro. Again, I would expect it to be able to handle it, but you never know.
2) Do you have the correct tail servo frequency/type selected in the gyro? If you're running the servo at lower than optimal freq it can impact response time. For instance, at freq 50hz (analog) each control cycle is .02s. At 333hz each cycle is .003s. So setting analog mode on a digital servo, while save in most instances, will result in substantially reduced performance.
3) In another thread a gyro setting for "delay" was mentioned. Usually this is used if the servo is inordinately slow (or tail blades are undersized, or the tail response is otherwise below expectations, also used for large helis simply due to inertia and the slower to react tail from steady state), so the gyro doesn't overshoot and cause "ringing" or oscillations in the tail control. Does your gyro have a similar setting for slow tail servos? Have you tried it? A 450/500 is on the border between a fast responding bird and slow responding bird for purposes of this setting, so using the larger heli setting may well calm this oscillation down. If these tests were with "no delay" or the small heli size setting, try the larger. If it was set larger, try the smaller setting. The last bit is important, if it was set for a large heli response but you have an "over performing" tail for that setting, the delay can result in this kind of oscillation too (tail moves faster than the setting anticipates, so it overshoots and the gyro has to correct back the other way, overshooting again, rinse repeat).
