How can I be sure this text appears on screen for at least one frame?

Here I have some code that flashes some text quickly up on the screen. It is important that it flashes like it does for aesthetic reasons. However, I've had a lot of trouble making it function consistently across computers with different framerates. Obviously, it's going to look slower on computers with slower framerates; there's not much we can do to help that, but it's important that it flashes for at least one frame so that the player has a chance to read it for gameplay purposes. On slower computers, I find the text appears very infrequently or hardly at all. Sometimes the sound plays denoting that the text should flash but it doesn't render for a visible frame. Why is the timing inconsistent across different framerates? And when it does flash, even though tremendously slower than with a high framerate, why does it sometimes miss rendering? How could these things be remedied? If anyone can shed some light on what I'm messing up I appreciate any help! Thanks!

 using UnityEngine;
 using System.Collections;
 using System;
 using UnityEngine.SceneManagement;
 
 public class terminalScreenGlitch : MonoBehaviour {
 
     public string[] possibleTexts;
     public Transform inputField;
     public bool visible = false;
     private Vector3 initPos;
     public float xDis = 1;
     public float yDis = 1;
     public float zDis = 1;
     public float jumpiness = 6;
     public float changeChance = 3;
 
     public float apearChance = 40;
     public int howOftenToTest = 6;
     private bool testAgain = true;
     public float apearTime = 0.2f;
 
     public bool goNow = false;
 
     void Start(){
         initPos = transform.position;
         StartCoroutine (testApear());
     }
 
     void Update()
     {
         if (goNow) {
             StartCoroutine (forceGo());
             goNow = false;
         }
 
         if (testAgain) {
             testAgain = false;
             StartCoroutine(testApear());
         }
 
         if (visible) {
             if (UnityEngine.Random.Range (0, changeChance) <= 1) {
                 inputField.GetComponent<TextMesh> ().text = "> " + possibleTexts [Mathf.RoundToInt (UnityEngine.Random.Range (0.0f, possibleTexts.Length - 1))];
             }
 
             if (UnityEngine.Random.Range (0, jumpiness) <= 1) {
                 transform.position = new Vector3 (initPos.x + UnityEngine.Random.Range (-xDis, xDis), initPos.y + UnityEngine.Random.Range (-yDis, yDis), initPos.z + UnityEngine.Random.Range (-zDis, zDis));
             }
 
         } else {
             inputField.GetComponent<TextMesh> ().text = "";
         }
 
     }
 
     IEnumerator testApear(){
         yield return new WaitForSeconds (howOftenToTest);
         if (UnityEngine.Random.Range (0, apearChance) <= 1) {
             visible = true;
             GetComponent<AudioSource> ().Play ();
             yield return null;
             yield return new WaitForSeconds (apearTime);
             visible = false;
             GetComponent<AudioSource> ().Stop ();
         }
         testAgain = true;
     }
 
     IEnumerator forceGo(){
         visible = true;
         GetComponent<AudioSource> ().Play ();
         yield return new WaitForSeconds (apearTime);
         visible = false;
         GetComponent<AudioSource> ().Stop ();
     }
 
 }