Why is my character's y axis randomly changing?

Okay, so I made a character, with a character controller. It's walking on a plane (the geometric figure of course). But what's really weird is that it randomly started looking like he's walking on a slightly bumpy terrain. What could be going wrong? Here's the script. (The other stuff is my gun system in progress) var speed = 1.0; var rotateSpeed = 3.0; var gunon = 0.0;

 function Update () 
 {
     if (Input.GetAxis("Vertical") && Input.GetAxis("Horizontal"))
         transform.Rotate(0, Input.GetAxis ("Horizontal") * rotateSpeed, 0);
         
    if (Input.GetAxis("Vertical") > 0.2)
    {
        var controller : CharacterController = GetComponent(CharacterController);
        animation.CrossFade("Walk");
        var forward = transform.TransformDirection(Vector3.forward);
        var curSpeed = speed * Input.GetAxis ("Vertical");
        controller.SimpleMove(forward * curSpeed);
     }
    else
    {
        if (gunon == 0.0)
             animation.CrossFade("Idle");
     }
     
     if (Input.GetAxis("Gun Pose"))
         gunon = 1.0;
     
     if (gunon == 1.0)
         animation.CrossFade("GunPose");
         if (Input.GetAxis("Gun Pose"))
             gunon = 0.0;
 }
 
 @script RequireComponent(CharacterController)