Orbital Camera Spazzing out uncontrollably..

Hey all, Im currently working on a 3rd person camera. Kind of like in WoW. Anyway, ive got no errors but as soon as i hit play, the camera spazzes out uncontrollably like it doesnt know where to go, and it shifts the position from what seems to be the min position allowance to the current position, every frame.

here is my code:

 using UnityEngine;
 using System.Collections;
 
 public class Third_Person_Camera : MonoBehaviour {
 
     public static Third_Person_Camera Instance;
     public Transform TargetLookAt;
     public float distanceCurr = 5f;
     public float distanceMin = 3f;
     public float distanceMax = 10f;
 
     public float distanceSmooth = 0.05f;
 
     public float xMouseSensitivity = 5f;
     public float yMouseSensitivity = 5f;
     public float mouseWheelSensitivity = 5f;
 
     public float xSmooth = 0.05f;
     public float ySmooth = 0.1f;
 
     public float yMinLimit = -40f;
     public float yMaxLimit = 80f;
 
 
     private float mouseX = 0f;
     private float mouseY = 0f;
     private float velX = 0f;
     private float velY = 0f;
     private float velZ = 0f;
     private float velDistance = 0f;
     private float startDistance = 0f;
 
     private Vector3 position = Vector3.zero;
 
     private Vector3 desiredPosition = Vector3.zero;
 
     private float desiredDistance = 0f;
 
 
     void Awake () {
         Instance = this;
     }
 
     void Start () {
         distanceCurr = Mathf.Clamp (distanceCurr, distanceMin, distanceMax);
         startDistance = distanceCurr;
         Reset ();
     }
     
     void LateUpdate () {
         if (TargetLookAt == null) {
             return;
         }
 
         HandlePlayerInput ();
         CalculateDesiredPosition ();
         UpdatePosition ();
     }
 
     void HandlePlayerInput () { 
         var deadZone = 0.01f;
 
         //this checks to see if the right mouse is down.
         //later check to see if also not over GUI.
         if (Input.GetMouseButtonDown (1)) {
             mouseX += Input.GetAxis("Mouse X") * xMouseSensitivity;
             mouseY -= Input.GetAxis("Mouse Y") * yMouseSensitivity;
         }
 
         //clamp or limit mouse rotation for Y-Axis.
         mouseY = camHelper.clampAngle (mouseY, yMinLimit, yMaxLimit);
 
         if (Input.GetAxis ("Mouse ScrollWheel") < -deadZone || Input.GetAxis ("Mouse ScrollWheel") > deadZone) {
             desiredDistance = Mathf.Clamp(distanceCurr - 
                                           Input.GetAxis ("Mouse ScrollWheel") * 
                                           mouseWheelSensitivity, 
                                           distanceMin, distanceMax);
         }
     }
 
     void CalculateDesiredPosition () { 
         //evaluate the players distance.
         distanceCurr = Mathf.SmoothDamp (distanceCurr, desiredDistance, ref velDistance, distanceSmooth);
 
         //then calculate the desired position..
         desiredPosition = CalculatePosition (mouseY, mouseX, distanceCurr);
     }
 
     Vector3 CalculatePosition (float rotationX, float rotationY, float distance) {
         Vector3 direction = new Vector3 (0, 0, -distance);
         Quaternion rotation = Quaternion.Euler (rotationX, rotationY, 0);
         return TargetLookAt.position + rotation * direction;
     }
 
     void UpdatePosition () { 
         
         var posX = Mathf.SmoothDamp (position.x, desiredPosition.x, ref velX, xSmooth);
         var posY = Mathf.SmoothDamp (position.y, desiredPosition.y, ref velY, ySmooth);
         var posZ = Mathf.SmoothDamp (position.z, desiredPosition.z, ref velZ, xSmooth);
 
         transform.position = new Vector3 (posX, posY, posZ);
 
         transform.LookAt (TargetLookAt);
     }
 
     public void  Reset () {
         mouseX = 0;
         mouseY = 10;
         distanceCurr = startDistance;
         desiredDistance = distanceCurr;
     }
 
     public static void UseExistingOrCreateNewMainCamera () {
         GameObject tempCamera;
         GameObject targetLookAt;
         Third_Person_Camera myCamera;
 
         if (Camera.mainCamera != null) {
             tempCamera = Camera.mainCamera.gameObject;
         } else { 
             tempCamera = new GameObject("Main Camera");
             tempCamera.AddComponent("Camera");
             tempCamera.tag = ("MainCamera");
         }
 
         tempCamera.AddComponent ("Third_Person_Camera");
         myCamera = tempCamera.GetComponent ("Third_Person_Camera") as Third_Person_Camera;
 
         targetLookAt = GameObject.Find ("targetLookAt") as GameObject;
 
         if (targetLookAt == null) {
             targetLookAt = new GameObject("targetLookAt");
             targetLookAt.transform.position = Vector3.zero;
         }
 
         myCamera.TargetLookAt = targetLookAt.transform;
     }
 }

the error has something to do with this:

I have isolated the issue to this:

         var posX = Mathf.SmoothDamp (position.x, desiredPosition.x, ref velX, xSmooth);
         var posY = Mathf.SmoothDamp (position.y, desiredPosition.y, ref velY, ySmooth);
         var posZ = Mathf.SmoothDamp (position.z, desiredPosition.z, ref velZ, xSmooth);
 
         transform.position = new Vector3 (posX, posY, posZ);

dont say its because on posZ one of the variable params should be zSmooth, cos thats not right.