Flickering materials/textures when changed by script

I am doing a serious game for a research project and I've run into some problems I can't get my head around.

What I'm trying to make is a display for an elevator that changes between different numbers over time to make it appear as the elevator is moving. I have tried different approaches, switching out textures, materials and even objects to make this display but they all end up the same, a seemingly random flickering between the different numbers.

I am new to scripting but this is what I've been trying to use. The script is supposed to change the material of the specified gameobject, wait for specified time, change the material again and so on.

 var timeBetweenFloors : float = 5;
 
 private var idDisplay : GameObject;
 
 function Awake () {    
     idDisplay = GameObject.Find('display');
 }
 
 function Update () {
     ElevatorRide();
 }
 
 function ElevatorRide () {
     idDisplay.gameObject.renderer.material = Resources.Load("display15") as Material;
     yield WaitForSeconds (timeBetweenFloors);
     idDisplay.gameObject.renderer.material = Resources.Load("display14") as Material;
     yield WaitForSeconds (timeBetweenFloors);
     idDisplay.gameObject.renderer.material = Resources.Load("display13") as Material;
     yield WaitForSeconds (timeBetweenFloors);
     idDisplay.gameObject.renderer.material = Resources.Load("display12") as Material;
 }

When I run the game this first changes the material of the display to the correct one, "display15", but on the next step it just begins to flicker between all of the mentioned materials in the code at a seemingly random rate.

The result is the same when I use .renderer.material.maintexture to switch out the texture instead. I also tried making a duplicate of the display object for each number and applied different textures to them and switched the rendering of these objects on and off as below but with the same flickering result.

 floor16.gameObject.renderer.enabled = false;
 floor15.gameObject.renderer.enabled = true;
 yield WaitForSeconds (TimeBetweenFloors);
 floor15.gameObject.renderer.enabled = false;
 floor14.gameObject.renderer.enabled = true;
 yield WaitForSeconds (TimeBetweenFloors);
 ...

Is there something I have done completely wrong in my code? Are the previously used materials/textures supposed to remain visible even after they are changed out?

Is there a better approach to make a display that changes texture/material with specified time intervals?