How do I clamp rotation with mouse look?

Hi, I'm trying to make a space sim game to mimic freelancer's controls.

Right now I've got everything working fine besides a glitch in my mouse look. I can use it to move around fine. However, if I move the mouse at the very top or bottom of the screen (ie. to make the ship dow a back flip) it would act all jittery. It can turn around allright if I do it horizontally (ie. move the mouse to the extreme left or extreme right), only From what I can tell there is something limiting the rotation in order to stop this. Hoever, I have no code for it whatsoever. This is the code for my mouse look:

 void DetermineRotation()
 {
  Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
  float hitdist = 1.0f;
  Vector3 targetPoint =  ray.GetPoint(hitdist);
 
  Quaternion targetRotation = Quaternion.LookRotation(targetPoint - rigidbody.position);
  rigidbody.MoveRotation(Quaternion.Slerp(rigidbody.rotation, targetRotation, turnSpeed * Time.deltaTime));
  }

It turns out I actually would rather it not do a backflip, however I would like to stop the jitter, any ideas? If thats not possible without some serious hard work, how do I make it allow backflipping?