Get a character to rotate instead of revolve

I am new to Unity and working on my first project. I want to make my character rotate as in spin, which it does if I take off gravity. When gravity is on and it is grounded, however, it revolves in a small circle around some invisible point. Here is my code:

 var rollSpeed = 6.0;
 
 var fastRollSpeed = 2.0;
 
 var jumpSpeed = 8.0;
 
 var gravity = 20.0;
 
 var rotateSpeed = 4.0;
 
 var duckSpeed = .5;
 
 
 private var moveDirection = Vector3.zero;
 
 private var grounded : boolean = false;
 
 private var moveHorz = 0.0;
 
 private var normalHeight = 2.0;
 
 private var duckHeight = 1.0;
 
 private var rotateDirection = Vector3.zero;
 
 private var isDucking : boolean = false;
 
 private var isBoosting : boolean = false;
 
 private var isJumping : boolean = false;
 
 var isControllable : boolean = true;
 
 var controller : CharacterController ;
 
 controller = GetComponent(CharacterController);
 
 function FixedUpdate() {
 
     if(!isControllable)
         Input.ResetInputAxes();
     else{
         if (grounded) {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= rollSpeed;
             
             
             if (Input.GetButton("Jump")) {
                 moveDirection.y = jumpSpeed;
                 isJumping = true;
                 }
             if(Input.GetButton ("Boost")) {
                 moveDirection *=fastRollSpeed;
                 isBoosting = true;
             }
             if(Input.GetButton("Duck")) {
                 controller.height = duckHeight;
                 controller.center.y = controller.height/2 + .25;
                 moveDirection *= duckSpeed;
                 isDucking = true;
             }
             if(Input.GetButtonUp("Duck")){
                 controller.height = normalHeight;
                 controller.center.y = controller.height/2;
                 isDucking = false;
             }
             if(Input.GetButtonUp("Boost")){
                 isBoosting = false;
             }
         }
         else
         {if(isJumping == true) {
             if(isBoosting == true){
                 if(Input.GetButton ("Jump")) {
                     moveDirection.y = jumpSpeed * 2;
                     isJumping = false;
                     isBoosting = false;
                     }
                 }
             }
         }
         moveHorz = Input.GetAxis("Horizontal");
         if (moveHorz > 0)
             rotateDirection = new Vector3(0, 1, 0);
         else if (moveHorz < 0)
             rotateDirection = new Vector3(0, -1, 0);
         else
             rotateDirection = new Vector3(0, 0, 0);
         moveDirection.y -= gravity * Time.deltaTime;
         
         var flags = controller.Move(moveDirection * Time.deltaTime) ;
         controller.transform.Rotate(rotateDirection * Time.deltaTime, rotateSpeed);
         grounded = ((flags & CollisionFlags.CollidedBelow) !=0 );
         }
     }