The prefabs stop instantiating after some time.

I have a script attached to an empty game object, that instantiates random car prefabs after waiting for 2 seconds. This script works fine, but after some time it stops instantiating the prefabs. It either stops after instantiating 36 objects or 72. Here is the code:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class CarMovement : MonoBehaviour {
 
     public GameObject audi;
     public GameObject black_viper;
     public GameObject car;
     public GameObject mini_truck;
     public GameObject mini_van;
     public GameObject taxi;
     public GameObject RandomCar;
 
     private List<GameObject> prefabList = new List<GameObject>();
 
     private int direction;
     private int vehicle;
 
     private Vector3 InstantiatingToNorth;
     private Vector3 InstantiatingToSouth;
     private Vector3 InstantiatingToLeft;
     private Vector3 InstantiatingToRight;
     private bool isInstantiating;
     private float InstantiateInterval;
 
     void Start () {
         isInstantiating = false;
         InstantiateInterval = 2.0f;
 
         prefabList.Add(audi);
         prefabList.Add(black_viper);
         prefabList.Add(taxi);
         prefabList.Add(car);
         prefabList.Add(mini_van);
         prefabList.Add(mini_truck);
     
         InstantiatingToNorth = new Vector3(1, 8, -1);
         InstantiatingToSouth = new Vector3(-1, -8, -1);
         InstantiatingToLeft = new Vector3(-10, 1, -1);
         InstantiatingToRight = new Vector3(10, -1, -1);
     }
 
     void FixedUpdate () {
 
         if(isInstantiating == false){
             InstantiateVehicle();
         }
 
     }
 
     void InstantiateVehicle(){
         isInstantiating = true;
         direction = Random.Range(0, 4);
         vehicle = Random.Range (0, 5);
 
 
         if (direction == 0){
 
             //Instantiate to north
             RandomCar = Instantiate(prefabList[vehicle], InstantiatingToNorth, Quaternion.Euler(0, 0, 180)) as GameObject;
             RandomCar.tag = "north";
             RandomCar.AddComponent("VehicleTranslation");
         
         } else if (direction == 1){
 
             //Instantiate to south
             RandomCar = Instantiate(prefabList[vehicle], InstantiatingToSouth, Quaternion.Euler(0, 0, 0)) as GameObject;
             RandomCar.tag = "south";
             RandomCar.AddComponent("VehicleTranslation");
 
         } else if (direction == 2){
 
             //Instantiate to left
             RandomCar = Instantiate(prefabList[vehicle], InstantiatingToLeft, Quaternion.Euler(0, 0, 270)) as GameObject;
             RandomCar.tag = "left";
             RandomCar.AddComponent("VehicleTranslation");
 
         } else if (direction == 3){
 
             //Instantiate to right
             RandomCar = Instantiate(prefabList[vehicle], InstantiatingToRight, Quaternion.Euler(0, 0, 90)) as GameObject;
             RandomCar.tag = "right";
             RandomCar.AddComponent("VehicleTranslation");
 
         }
         if (InstantiateInterval > 1.0f){
 
             StartCoroutine(WaitFor(InstantiateInterval));
             InstantiateInterval -= (InstantiateInterval * Time.deltaTime);
 
         } else {
             InstantiateInterval = 1.0f;
         } 
     }
 
     IEnumerator WaitFor(float duration){
         yield return new WaitForSeconds(duration);
         isInstantiating = false;
     }
 
 }

The script attached to each object, also destroys the object after the car has reached or passed that coordinate. Any help with this will be helpful.