Trying to create gameobjects at intervals [C#]

Hey guys,
my current challenge is trying to instantiate "enemy" gameobjects at 2 second intervals. However, when I try to use the "waitForSecond" type function, they are simply instantiated MANY TIMES and nearly crashes unity. Ideally, my (pseudo)code would look like this:

void Update()
{
waitForSeconds(2);
gameObject.instantiate("Enemy");
}

 

that is what i use for the moment

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class WaveSpawningScript : MonoBehaviour {
5.  
6.     public  GameObject     spawn_point;
7.     public  int         wave_size     = 5;
8.     public     int            mul = 1;
9.     private float         spawn_rate     = 2.5f;
10.     private float         next_spawn     = 0.0f;
11.  
12.     public void Update_()
13.     {
14.         if(Time.time >= next_spawn && wave_size != 0)
15.         {
16.             SpawnMonster();
17.         }
18.     }
19.  
20.     private void SpawnMonster()
21.     {
22.  
23.             next_spawn =  Time.time + spawn_rate;
24.             wave_size --;
25.             GameObject monster = Instantiate(Resources.Load("Monster/Slime/SlimeMonster"),spawn_point.transform.position,Quaternion.AngleAxis(180,new Vector3(0,1,0))) as GameObject;
26.             monster.GetComponent<Monster>().life = 20 * mul;
27.     }
28. }
29.  

Update_ is called from the main loop when needed

 

Code (csharp):

1. InvokeRepeating

 

Thanks for the help, guys!