My Y position goes fixed to some value

I have made battle arena in which I have put Box Collider( no trigger included ), Mesh Collider( triger included, convex included), Terrain Collider(triger included) and two object vehicles( Rigidbody and Mesh Collider ) and along it I have made wander script. Now what I want here is for objects to move along the terrain, however every time I try, it either falls off the terrain or go at some y rate which is +2 in units higher that it is supposed to be( it is 3 valued, and it goes up to 5.000019 without any reason ). So, I am pleading you to discuss this matter, in order to solve the possible cause of it. As far as I know, the scripts behavior is out of question for this issue.

 #pragma strict
 
 var Speed= 2;
 var wayPoint : Vector3;
 
 function Start(){
    //initialise the target way point
    Wander();
 }
 
 function Update() 
 {
    // this is called every frame
    // do move code here
    transform.position += transform.TransformDirection(Vector3.forward)*Speed*Time.deltaTime;
     if((transform.position - wayPoint).magnitude < 3)
     {
         // when the distance between us and the target is less than 3
         // create a new way point target
         Wander();
 
         // don't need this 
 
         //transform.LookAt(wayPoint);
         //transform.position += transform.TransformDirection(Vector3.forward)*Speed*Time.deltaTime;
     }
 }
 function Wander()
 { 
    // does nothing except pick a new destination to go to
     wayPoint= Vector3(Random.Range(0, 300),3, Random.Range(0, 300));
    // don't need to change direction every frame seeing as you walk in a straight line only
     transform.LookAt(wayPoint);
     Debug.Log(wayPoint + " and " + (transform.position - wayPoint).magnitude + gameObject.name);
 }