Hmm...what way is the tail rotor going? Is the tail rotor fastened securely on the shaft? Sometimes if you crash or something it might get loose because that little silicon rubber holder shifts, and that will usually cause the tail rotor to barely spin. If you're going to the left too much, keep moving that proportional mixing to the right, and if it's going right then vice versa. Try to get to the point where it keeps going the opposite direction it went before, and then back off a bit and you should have it. If your tail's twiching (jerking left and right and back again, this should be pretty obvious) then tune down the gyro gain switch. There's not much else I can suggest, maybe check the motor plugs to see if they're secure?

Also, can you get off the ground ok? Or does it just stay on the ground and slide around?