As you noticed it was going in, did you note if you had any movement of the other control surfaces.

I know that when something like this occurs, it's next to impossible to pay attention to anything else, but it could help eliminate the radio gear.

e.g. a bad servo or a disconnected servo.

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I had a Hitec servo recently that would work perfectly on the ground and usually in the air.

However when the engine hit a certain RPM the servo ( which was on the tail away from the engine ) would go nuts.

At first I though that this was flutter or due to excessive speed.

All interconnects were fine and if I reversed the connections ( I had two servos on the tail ) the problem remained with the servo.

Swapping it out for a new one fixed the problem.