Fixing Diagnol Speed Boost C#

I am running into a problem where a function is being called over and over again, however I only want it to be called once each time an input is detected. That function name is DiagFix that you can see in my code below

 using UnityEngine;
 using System.Collections;
 
 
 
 public class movement : MonoBehaviour {
     
     
     public GameObject GamePiece;
     public float speed = 0.4f;
     public bool dinitiated = false;
     
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
         
         if ((Input.GetButton ("Forward") && Input.GetButton ("SLeft")) || 
             (Input.GetButton ("Forward") && Input.GetButton ("SRight")) ||
             (Input.GetButton ("Backward") && Input.GetButton ("SLeft")) || 
             (Input.GetButton ("Backward") && Input.GetButton ("SRight"))) {    
         
             DiagFix();
         }
         
         Vector3 minusx = new Vector3(-speed,0,0);
         Vector3 plusx = new Vector3(speed,0,0);
         Vector3 minusz = new Vector3(0,0,-speed);
         Vector3 plusz = new Vector3(0,0,speed);
         
         if (Input.GetButton ("Forward")) {
             if (GamePiece.transform.position.z < 24.5f) {
                     GamePiece.transform.position += plusz; 
             }
         }
         if (Input.GetButton ("Backward")) {
             if (GamePiece.transform.position.z > 0.5f) {
                     GamePiece.transform.position += minusz; 
             }
         }
         if (Input.GetButton ("SLeft")) {
             if (GamePiece.transform.position.x > 0.5f) {
                     GamePiece.transform.position += minusx; 
             }
         }
         if (Input.GetButton ("SRight")) {
             if (GamePiece.transform.position.x < 24.5f) {
                     GamePiece.transform.position += plusx; 
             }
         }
     }
     
     void DiagFix () {
         if( dinitiated = false ) {
             dinitiated = true;
             speed = speed / 4;
         } else {