The Frog Moves, Turns, Then Seems to Slow Down and Stop?

private Animator playerAnimator;
private float moveHorizontal;
private float moveVertical;
private Vector3 movement;
private float turningSpeed = 20f;
private Rigidbody playerRigidbody;


// Use this for initialization
void Start () {
    // Gather input from the keyboard
    playerAnimator = GetComponent<Animator> ();
    playerRigidbody = GetComponent<Rigidbody> ();

}

// Update is called once per frame
void Update () {
    moveHorizontal = Input.GetAxisRaw("Horizontal");
    moveVertical = Input.GetAxisRaw("Vertical");

    movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);


}

void FixedUpdate () {
        // If the player movement doesn't equal zero, play the script
    if (movement != Vector3.zero) {
            // Will create target rotation based on movement vector
        Quaternion targetRotation = Quaternion.LookRotation(movement, Vector3.up);
            // And create another rotation that represents turning
        Quaternion newRotation = Quaternion.Lerp(playerRigidbody.rotation, targetRotation, turningSpeed * Time.deltaTime);
            // And change the player's rotation to the new incremental rotation
        playerRigidbody.MoveRotation(newRotation);
        playerAnimator.SetFloat ("Speed", 3f);
    } else {
        playerAnimator.SetFloat ("Speed", 0f);


    }
}