Camera behaviour when looking directly up or down

Hi I have a camera in my project that looks at a 3D cursor projected into the world... I'm using this code to track it:

     Vector3 relPos = lookatObject.transform.position - transform.position;
     Quaternion curRot = transform.rotation;
     Quaternion tarRot = Quaternion.LookRotation(relPos);
     transform.rotation = Quaternion.Lerp(curRot, tarRot, turnSpeed * Time.deltaTime);

lookatObject being the 3D cursor. When I look left or right the camera smoothly rotates around 360 degrees but when I look up or down the camera starts going mad (not sure how to better explain it lol) and refuses to rotate further than 90 degrees around X (I don't mind if the camera rotates to be inverted since this is a space game). This is the code that projects the cursor if it helps.

         Vector3 position = new Vector3(Input.mousePosition.x, Input.mousePosition.y, 100);
         position = Camera.main.ScreenToWorldPoint(position);
         transform.position = position;

Hope someone can help, thanks!