skigame movement problem

hello. i am working on a ski game but i got a little smoothness problem whit my movementscript please jump over a few hills and see it yourself:click here

as you could see: if you make a big jump its shaking your whole character

PS: this is not the game it self just a little scene i made for my question

this is the script:

     var speed : float = 10.0;
     var jumpSpeed : float = 8.0;
     var gravity : float = 10.0;
     private var moveDirection : Vector3 = Vector3.zero;
     var tracker: Transform;
     var rotateSpeed = 3.0;
     
 
         
         function Update ()
             {
             
                 var controller : CharacterController = GetComponent(CharacterController);
                 transform.Rotate(0, Input.GetAxis ("Horizontal") * rotateSpeed, 0);
                 var forward = transform.TransformDirection(Vector3.forward);
                 var curSpeed = speed * Input.GetAxis ("Vertical");
                 controller.SimpleMove(forward * curSpeed);            
         
                 var hit1: RaycastHit;
         var hit2: RaycastHit;
         
         if (Physics.Raycast(tracker.position, -Vector3.up, hit1)) {
             if (Physics.Raycast(tracker.TransformPoint(Vector3.forward * 0.1), -Vector3.up, hit2)) {
                 transform.rotation = Quaternion.LookRotation(hit2.point - hit1.point, hit1.normal);
     }                
                             if (controller.isGrounded) {
             
             moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
                                     Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
     
     
     
             if (Input.GetButton ("Jump")) {
                 moveDirection.y = jumpSpeed;
                 }                
           }
     
           
   
         controller.Move(moveDirection * Time.deltaTime);    
         }        
     
             }
             function FixedUpdate () {
         rigidbody.AddForce (-Vector3.up * 10);    
     
     }
         @script RequireComponent(CharacterController)

please. can someone fix this problem?