Invoke Invoking once?

Hello,

I'm having a weird issue with the invoke command. It seems that the delay on the invoke works once, then the materials just start randomizing per frame as if there was no delay. Is there something about invoke that I'm not getting?

Thanks! Grimm

 using UnityEngine;
 using System.Collections;
 
 public class SCRIPT_MaterialRandomizer : MonoBehaviour {
     
     public UnityEngine.Material Material_1;
     public UnityEngine.Material Material_2;
     public UnityEngine.Material Material_3;
     public UnityEngine.Material Material_4;
     public int ExecDelay = 1;
     
     public ArrayList MaterialArray = new ArrayList();
     
     // THIS ASSIGNS A RANDOM MATERIAL TO THE BLOCK FROM THE ARRAY OF MATERIALS
     void RandomizeMaterial() {
         int Rndm_MatID = Random.Range(0, MaterialArray.Count);
         
         renderer.material = MaterialArray[Rndm_MatID] as UnityEngine.Material;
     }
     
     // THIS DELAYS EXECUTION FOR (ExecDelay) SECONDS THEN CALLS THE MATERIAL RANDOMIZER FUNCTION
     void DelayUpdate() {
         RandomizeMaterial();
     }
     
 
     // Use this for initialization
     void Start () {
         MaterialArray.Add(Material_1);
         MaterialArray.Add(Material_2);
         MaterialArray.Add(Material_3);
         MaterialArray.Add(Material_4);
         
         RandomizeMaterial();
     }
     
     // Update is called once per frame
     void Update () {
         //StartCoroutine(DelayUpdate());
         //Invoke("DelayUpdate", 10);
         InvokeRepeating("DelayUpdate", 1, 10F);
     }
 }