3d Character rotates in an arc rather than on the spot [video attached]

I've been stuck on this problem for a while and haven't found anyone having similar difficulties. Basically I have a 3rd person character that I am trying to get to move and rotate at the same time. My character moves and rotates to begin with OK, but eventually the character starts to rotate in huge arcs which you can see in the attached video. Any help on this would be appreciated as I've spent too many hours now without getting anywhere.

Here is the video

and here is my code

      public float speed = 2.0F;
      public float jumpSpeed = 1.0F;
      public float gravity = 50.0F;
      public float turnSmoothing = 15f;
      public float lookSpeed = 100;
  
      public Vector3 moveDirection;
  
      public float moveHorizontal;
      public float moveVertical;
  
      public int grounded = 1;
      public Transform head;
      public Transform groundcheck;
  
  
      float GroundDistance = 0.5f;
  
      bool IsGrounded()
      {
          return Physics.Raycast(groundcheck.position, -Vector3.up, GroundDistance);
      }
  
      void Update()
      {
  
          moveHorizontal = Input.GetAxisRaw("Horizontal");
          moveVertical = Input.GetAxisRaw("Vertical");
  
          moveDirection = new Vector3(moveHorizontal, 0, moveVertical);
          moveDirection = Camera.main.transform.TransformDirection(moveDirection);
  
          ConstantForce headcf = head.GetComponent<ConstantForce>();
  
          CharacterController controller = GetComponent<CharacterController>();
  
          if (controller.isGrounded)
          {
              
              if (Input.GetButton("Jump"))
              {
                  if (IsGrounded() == true)
                  {
                      moveDirection.y = jumpSpeed;
                      headcf.force = new Vector3(0, 180, 0);
                      print("Ground");
                      grounded = 1;
                  }
                  else
                  {
                      headcf.force = new Vector3(0, 60, 0);
                      print("Not Ground");
                      grounded = 0;
                  }
                  
              }
  
          }
  
          moveDirection.y -= gravity * Time.deltaTime;
          controller.Move(moveDirection * Time.deltaTime);
  
          if (moveHorizontal != 0f || moveVertical != 0f)
          {
              Rotating(moveHorizontal, moveVertical);
          }
      }
  
      void Rotating(float mh, float mv)
      {
  
          Vector3 targetDirection = new Vector3(mh, 0f, mv);
  
          Quaternion targetRotation = Quaternion.LookRotation(moveDirection, Vector3.up);
  
          moveDirection.y = 0f;
  
          Quaternion newRotation = Quaternion.Lerp(transform.rotation, targetRotation, turnSmoothing * Time.deltaTime);
  
          transform.rotation = newRotation;
      }