Walls not moving in

For my game I need the outer walls to close in towards the center, but for some reason they just hop forward into a close position instantly, then all collide in the center and take off in random directions. Help?

 using UnityEngine;
 using System.Collections;
 
 public class ApproachWalls : MonoBehaviour {
 
     public Vector3 pos; 
     public Vector3 newPos;
     public Vector3 differenceOne = new Vector3(0,0,5);
     public Vector3 differenceTwo = new Vector3(0,0,5);
     public Vector3 differenceThree = new Vector3(5,0,5);
     public Vector3 differenceFour = new Vector3(-5,0,5);
     // Use this for initialization
     void Start () {
         pos = transform.position;
         InvokeRepeating ("closeIn", 5.0f, 5.0f);
     }
     
     // Update is called once per frame
     void Update () {
 
 
     }
 
     void closeIn (){
                 if (transform.position.z < 0 && transform.position.x == 0 && transform.position.z !=0) {
                     newPos += differenceOne;
                     rigidbody.MovePosition(newPos);
 
         } else if (transform.position.z > 0 && transform.position.x == 0 && transform.position.z !=0) {
                     newPos += differenceTwo;
                     rigidbody.MovePosition(newPos);
 
         } else if (transform.position.x < 0 && transform.position.z == 0 && transform.position.x !=0) {
                     newPos += differenceThree;
                     rigidbody.MovePosition(newPos);
         
         } else if (transform.position.x > 0 && transform.position.z == 0 && transform.position.x !=0) {
                     newPos += differenceFour;
                     rigidbody.MovePosition(newPos);
                 }
         }
     void OnCollisionEnter (Collision col){
             Physics.IgnoreCollision(col.collider, collider);
                 
         }
     }