Why Does My Prefab Fly Upwards When Moving Around?

So I'm making an overly-simple FPS, and below is the zombie movement script. They're simplistic bodies using joints and limbs to almost resemble a human. However, upon spawning, my zombies constantly fly straight upwards, reach a max height, fall down to the terrain and bounce back up. I know it's a problem with the GroundToTerrain() below, but I can't seem to figure out what it is.

Does anybody have a good idea?

 var speed : float = 3.0f;
 var bodyHeight : float = 4.0f;
 
 function Start () {
     GroundToTerrain();
     rigidbody.constraints = RigidbodyConstraints.FreezeRotationX | RigidbodyConstraints.FreezeRotationZ;
 }
 
 function Update () {
     GroundToTerrain();
     rigidbody.velocity = Vector3.forward * speed;
 }
 
 function GroundToTerrain(){
     var hit : RaycastHit;
     Physics.Raycast(transform.position, -transform.up, hit );
     transform.position.y = hit.point.y + bodyHeight;
 }