Locking rotation makes object spin

Hey,

I'm trying to use the keybaord to rotate the camera around an object, And I dont want to rotate the camera around the z axis

here is my code:

 float h = Input.GetAxis("Horizontal");
 float v = Input.GetAxis("Vertical");

 Vector3 rotationAxis = new Vector3(v, h, 0);

 _cam.transform.RotateAround(_rotationOrigin, rotationAxis, _baseMoveSensitivity * Time.deltaTime);

 _cam.transform.rotation = Quaternion.Euler(_cam.transform.rotation.eulerAngles.x, _cam.transform.rotation.eulerAngles.y, 0);

My problem is that although I dont rotate around the z axis, the camera spins when I move over the object. Is there a way to fix it?
Or do it in another way that will not cause that spin to occur?

Thanks!