IF the gain is too high, then the tail will oscillate very fast and violently. If it's too low, it will do what is called hunting, which is a very slow wag.
I"m not sure what Tx you are using, but make sure you have the gyro set to channel 5 and that you are in the positive numbers, or above 50 if you have a DX6i. I start out at 40 or 65 respectively.