FP Player Object flying out of control on start

private CharacterController charControl;
[SerializeField]
private float moveSpeed;
private bool isGrounded;
private Rigidbody playerRigidBody;
[SerializeField]
private float jumpHeight;

// Use this for initialization
void Start () {

    charControl = GetComponent<CharacterController>();
    isGrounded = true;
    playerRigidBody = GetComponent<Rigidbody>();

}

// Update is called once per frame
void Update () {

    MovePlayer();

    if (isGrounded)
    {
        if (Input.GetKeyDown("Jump"))
        {
            playerRigidBody.velocity = new Vector3(0f, jumpHeight, 0f);
            isGrounded = false;
        }
    }

}

void MovePlayer()
{
    float horizontal = Input.GetAxisRaw("Horizontal");
    float vertical = Input.GetAxisRaw("Vertical");

    Vector3 moveDirectionSide = transform.right * horizontal * moveSpeed;
    Vector3 moveDirectionForward = transform.forward * vertical * moveSpeed;

    charControl.SimpleMove(moveDirectionSide);
    charControl.SimpleMove(moveDirectionForward);

}

private void OnCollisionEnter(Collision collision)
{

    collision.gameObject.CompareTag("Ground");
    isGrounded = true;

}

[SerializeField]
private float mouseSensititvity;
[SerializeField]
private Transform playerBody;

float xAxisClamp = 0;

void Start()
{
    //Lock the cursor in the center of the screen
    //To allow the game to be minimized while also keeping the mouse...
    //...locked after maximizing, this needs to be in the update method
    Cursor.lockState = CursorLockMode.Locked;
}

private void Update()
{
    RotateCamera();
}

void RotateCamera()
{
    //Get the mouse location
    float mouseX = Input.GetAxis("Mouse X");
    float mouseY = Input.GetAxis("Mouse Y");

    //The amount the camera should rotate
    float rotateAmountX = mouseX * mouseSensititvity;
    float rotateAmountY = mouseY * mouseSensititvity;

    xAxisClamp -= rotateAmountY;

    //Get the current rotation of the camera and player gameObject...
    Vector3 targetRotationCamera = transform.rotation.eulerAngles;
    Vector3 targetRotationBody = playerBody.rotation.eulerAngles;

    //...add the appropriate amount the camera and body need to rotate...
    targetRotationCamera.x -= rotateAmountY;
    targetRotationBody.y += rotateAmountX;

    //Make sure the camera doesn't flip over when looking too far down or up!
    targetRotationCamera.z = 0;

    if(xAxisClamp > 90)
    {
        xAxisClamp = 90;
        targetRotationCamera.x = 90;
    }

    if(xAxisClamp < -90)
    {
        xAxisClamp = -90;
        targetRotationCamera.x = 270;
    }

    //...and put it back into the respective transforms
    transform.rotation = Quaternion.Euler(targetRotationCamera);
    playerBody.rotation = Quaternion.Euler(targetRotationBody);

}