I lose control of my ball?

Let me be more specific...

I'm making a game where you have to move the ball through a maze and pick up points on the way. To control the ball, I am using this code here...

 #pragma strict
 
 var rotationSpeed = 200;
 var jumpHeight = 8;
 
 private var isFalling = false;
 
 function Update () 
 {
     // Handle ball rotation along X azis.
     var rotationX : float = Input.GetAxis("Horizontal") * rotationSpeed;
     rotationX *= Time.deltaTime;
     rigidbody.AddRelativeTorque(Vector3.back * rotationX);
     
     // Handle ball rotation along Z axis.
     var rotationZ : float = Input.GetAxis("Vertical") * rotationSpeed;
     rotationZ *= Time.deltaTime;
     rigidbody.AddRelativeTorque(Vector3.right * rotationZ);
     
     // Controls jumping.
     if (Input.GetKey(KeyCode.Space) && isFalling == false)
     {
         rigidbody.velocity.y = jumpHeight;
     }
     isFalling = true;
 }
 
 function OnCollisionStay()
 {
     isFalling = false;
 }

Whenever I try to control my ball, one axis will work absolutely fine but when ever I try to control the other axis, the ball loses control and I can't stop the ball as I can't control the rotations. If anyone can tell me what has gone wrong and how to fix it, I would be very grateful. Thank you in advanced :)