Movement script flips character upside down

I made the following script, which applies forward movement, while rotating the character, to simulate WSAD movement.

      void ApplyRotation(float h, float v)
      {
          //We need to be able to turn around the Y axis using the arrow/WASD keys,
          //so we'll make a rotation that enables this.
          float turnVal = (h == 0f) ? v : h;
          tiltZ = Utilities.ClampAngle(tiltZ, 0f, 360f) + turnVal;
          Vector3 lv = new Vector3(h, 0, v);
          var nrot = Quaternion.LookRotation(lv,transform.up);
          transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.Euler(transform.rotation.eulerAngles + nrot.eulerAngles), Time.deltaTime * turningSpeed);
      }

It works just fine at first, but as the player keeps moving, they are slowly flipped upside down. I have tracked the bug to this script, but am unable to find what is wrong with it. Anyone have any ideas?

PS: The code for Utilities.ClampAngle() is below.

 public static float ClampAngle(float angle, float minAngle, float maxAngle)
     {
         int full_rotations = (angle > 360f || angle < -360f) ? Math.Abs(Convert.ToInt32(angle)) % 360 : 0;
 
         //We need to either subtract (in case angle is positive) or
         //add (in case angle is negative) any full rotations (360 degrees)
         //before we clamp.
         //This is equivalent to:
         //do
         //{
         //  if(angle < -360)
         //      angle += 360f;
         //  if(angle > 360)
         //      angle -= 360f;
         //} while (angle < -360f || angle > 360f);
         if (full_rotations > 0)
             angle += Mathf.Sign(angle) * (-1) * full_rotations * 360f;
 
         return Mathf.Clamp(angle, minAngle, maxAngle);
     }