Normal Walker: Player Y Rotation Snapping back to 0?

I found a script recently that allows you to walk on an object and rotate based on the normals below the object. The problem is that the player y rotation snaps back to 0 whenever the rotation of the player is modified. This is a problem as I am making a first person game where you navigate the inside of a tube. Also how would I make the transition smoother when the player rotation changes. I know I would need to add a slerp or lerp of some sort but I am a noob and am not familiar with quaternions.

 using UnityEngine;
 using System.Collections;
 
 
 
 public class NormalWalker : MonoBehaviour
 {
     //this game object's Transform  
     private Transform goTransform;
 
     //the speed to move the game object  
     private float speed = 6.0f;
     //the gravity  
     private float gravity = 50.0f;
 
     //the direction to move the character  
     private Vector3 moveDirection = Vector3.zero;
     //the attached character controller  
     private CharacterController cController;
 
     //a ray to be cast   
     private Ray ray;
     //A class that stores ray collision info  
     private RaycastHit hit;
 
     //a class to store the previous normal value  
     private Vector3 oldNormal;
     //the threshold, to discard some of the normal value variations  
     public float threshold = 0.009f;
 
     // Use this for initialization  
     void Start()
     {
         //get this game object's Transform  
         goTransform = this.GetComponent<Transform>();
         //get the attached CharacterController component  
         cController = GetComponent<CharacterController>();
     }
 
     // Update is called once per frame  
     void Update()
     {
         //cast a ray from the current game object position downward, relative to the current game object orientation  
         ray = new Ray(goTransform.position, -goTransform.up);
 
         //if the ray has hit something  
         if (Physics.Raycast(ray.origin, ray.direction, out hit, 5))//cast the ray 5 units at the specified direction    
         {
             //if the current goTransform.up.y value has passed the threshold test  
             if (oldNormal.y >= goTransform.up.y + threshold || oldNormal.y <= goTransform.up.y - threshold)
             {
                 //set the up vector to match the normal of the ray's collision  
                 goTransform.up = hit.normal;
             }
             //store the current hit.normal inside the oldNormal  
             oldNormal = hit.normal;
         }
 
        
 
     }
 }