unity3d: making the AI jump over obstacle with a tag

hi,

I have a simple AI which just goes forward. In front of the AI there is an obstacle (just a cube) with a tag "terrain". What i want my AI to do is jump over it. Here is what i have:

 #pragma strict
 
 var Speed : float = 2;
 var jumpSpeed : float = 8.0;
 var vSpeed : float = 0.0;
 
 function Awake () {
     myTransform = transform;
 }
 
 function Update () {
 
     if(jump == true){
         vSpeed = jumpSpeed;
     }
     if(jump == false){
         vSpeed = 0;
     }
     // not a very smooth jump but probably good enough
     myTransform.position += myTransform.up * vSpeed * Time.deltaTime;
     // move forward
     myTransform.position += myTransform.forward * Time.deltaTime * Speed;
 
 }
 
 function OnCollisionEnter(col : Collision){
     if(col.gameObject.tag == "terrain"){
         jump = true;
     }
 }

 // i don't think this part is working
 function OnCollisionExit (col : Collision) {
     if(!col.gameObject.tag == "terrain"){
         jump = false;
     }
 }

My AI does jump over the obstacle but then keeps jumping and i cant find a way to stop it from jumping

plz help

And thx in advance