How to get User Input to Work in OnGUI and Update Functions Please help!

Ok so I have been trying to fix this for 2 days, thinking my logic somewhere was wrong. But after a little test statement I realized its the way I'm taking user input. It is happening too fast from what I can see. If someone could explain a better way to take the input so that the menu is not skipping to the end when I hold down the Z key and then when I release it, it skips to the start of dialogue again.... Any suggestions? thanks :)

Where I am changing the user input dialogue window based on the user input of Z is the key part of this. The rest seems to work fine.

Update Function.

     void Update () {
 
         if (Input.GetKeyDown (KeyCode.X)) {
             moveWindowDisabled = true;
         } else if (Input.GetKeyUp (KeyCode.X)) {
             moveWindowDisabled = false;
         }
         if (Input.GetKey (KeyCode.Z)) {
             dialogueWindowContinue = true;
         } else if (!Input.GetKey (KeyCode.Z)) {
             dialogueWindowContinue = false;
         }
     }

OnGUI Function:

     void OnGUI () {
 
         if (moveWindowDisabled && moveWindow.gameObject.activeInHierarchy) {
             moveWindow.gameObject.SetActive(false);
         }
 
         if (TurnBasedCombatStateMachine.playersChoice && !dialogueWindow.gameObject.activeInHierarchy) {
             dialogueWindow.gameObject.SetActive (false);
         } else if (!TurnBasedCombatStateMachine.playersChoice) {
             dialogueWindow.gameObject.SetActive (true);
         }
 
         dialogueWindowText.text = "It was a Critical Hit ...";
         if (dialogueWindowContinue){
             dialogueWindowText.text = "Status Effects and Damage Calculations next....";
             if (dialogueWindowContinue){
                 dialogueWindowText.text = "Testing testing 123......";
             }
         }
 
 
         /*if (BattleStateStart.playerStartedFirst) {
             PlayerFirstDialogue ();
         } else if (!BattleStateStart.playerStartedFirst) {
             EnemyFirstDialogue ();
         }*/
 
     }