Gimbal lock - object moving on surface of sphere

Hello,

I have an object moving on the surface of a sphere, but it gets stuck in what looks like gimbal lock when it reaches the north pole...

I thought my code should avoid this, as it's using all quaternions... but I've obviously messed something up somewhere :-/

here's what I've got so far:

 var relativePos = transform.position - Vector3.zero;
 var rotation = Quaternion.LookRotation(-relativePos);
 rotation *= Quaternion.AngleAxis(-90, Vector3.right);
 transform.rotation = rotation;
     
 if (Physics.Raycast (transform.position, -transform.up, groundHit, 100)) {
     transform.position = groundHit.point * 1.01;
     transform.position += (transform.forward * speed * Time.deltaTime);
 }