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My friend had a issue similar to this. 2 things i would check. First is the tail out shaft and gear. The output shaft and gear a basically molded together and in a crash if you hit the tail can shift the gear on the shaft. Even a little bit of play can affect it. Pull out and in on the shaft and see how much it moves. The other is the tail rotor hub. If you take the hub off and hold the grips and rock them back and forth and the hub spins you might have a bent hub.
Hope this helps,
Stratus 90, Synergy E6, Trex 450 Pro