What is wrong with my RotateAround?

Hey all!

I wrote this script and i attached to my camera:

 void Start()
 {
     if (rigidbody)  rigidbody.freezeRotation = true;
 
     cursorPosition = new Vector2(Input.GetAxis("Mouse X"), Input.GetAxis("Mouse Y"));
     cursorDiff = new Vector2(0.0f, 0.0f);
 }
     
 void UpdateCursorDiff()
 {
     cursorDiff = cursorPosition - new Vector2(Input.GetAxis("Mouse X"), Input.GetAxis("Mouse Y"));
     cursorPosition = new Vector2(Input.GetAxis("Mouse X"), Input.GetAxis("Mouse Y"));
 }
 
 void UpdateLookDirection()
 {
     characterHead.transform.RotateAround(new Vector3(0.0f, 1.0f, 0.0f), -(cursorDiff.x * Time.smoothDeltaTime) * cursorSensitivity);
     characterHead.transform.RotateAround(new Vector3(1.0f, 0.0f, 0.0f), (cursorDiff.y * Time.smoothDeltaTime) * cursorSensitivity);
     characterHead.transform.eulerAngles = new Vector3(characterHead.transform.eulerAngles.x, characterHead.transform.eulerAngles.y, -90.0f);
 }
 
 void Update()
 {
     UpdateCursorDiff();
     UpdateLookDirection();
 }

But my character's head vibrates and when i move my mouse his head turns back to its original state... Can anyone help me?

(Sorry for my bad english! I am hungarian.)