Problem With Wandering Script and Terrain.SampleHieght

I have been trying to get a basic wandering script done for a project I have been working on. I got everything to work except my terrain.SampleHieght, when even I add it in it causes my object to go off indefinitely in one direction along the Z axis. Any advice is helpful and appreciated.

 #pragma strict
 var Speed = 3;
 private var wayPoint : Vector3= Vector3.zero;
 var circleSize = 20;
 //variables speed, circlesize, and range of Vectors for waypoint
 
 function Start()
 {
     random();
 }
 
 function Update ()
 {
 transform.position += transform.TransformDirection(Vector3.forward)*Speed*Time.deltaTime;
 
 
 
 
 if ((transform.position - wayPoint).magnitude < 1)
 {
 //movement changes direction on approching a certain distance ot the waypoint
 random();
 } 
 }
 function random (){
 
 var chooseWayPoint : Vector3 = Random.insideUnitSphere * (circleSize);
 wayPoint.y = Terrain.activeTerrain.SampleHeight(wayPoint)+ Terrain.activeTerrain.GetPosition().y;
 wayPoint = new Vector3 (chooseWayPoint.x, wayPoint.y, chooseWayPoint.z);
 
 //set the sphere of allowed movement
 transform.LookAt(wayPoint);
 
 }