Unity Mouse Look Script Issue

My mouse look script would work perfectly, if the camera wouldn't rotate on the Z axis. My script is:

 public float xSensitivity = 10f;
 public float ySensitivity = 10f;
 public float minHeight = -360f;
 public float maxHeight = 360f;

 float rotationX = 0f;

 float rotationY = 0f;

 void Update() {

     rotationX = Input.GetAxis ("Mouse X") * xSensitivity / 20;
     rotationX = ClampAngle (rotationX, minHeight, maxHeight);
     Quaternion xQuaternion = Quaternion.AngleAxis (rotationX, transform.up);
     transform.rotation = transform.localRotation * xQuaternion;

     rotationY = Input.GetAxis ("Mouse Y") * ySensitivity / 20;
     rotationY = ClampAngle (rotationY, minHeight, maxHeight);
     Quaternion yQuaternion = Quaternion.AngleAxis (rotationY, (transform.right * -1));
     transform.rotation = transform.localRotation * yQuaternion;

     Quaternion regRotation = transform.rotation;
     transform.rotation = regRotation * xQuaternion * yQuaternion;

 }

 public static float ClampAngle(float angle, float min, float max) {
     if( angle <= -360f) {
         angle += 360f;
     }
     if (angle >= 360f) {
         angle -= 360f;
     }
     return Mathf.Clamp (angle, min, max);
 }

I don't know how to keep it from rotating on that axis. Any ideas on how to fix it?