Why do the characters jitter at some places?

I created

http://volksglauben.net46.net/test/Weltkugel/Weltkugel.html (Use WASD to rotate, SCROLLUP/DOWN to zoom)

using this script for characters to walk around the planet

 using UnityEngine;
 using System.Collections;
  
 public class OrbitPlanet : MonoBehaviour 
 {
     public float rotationSpeed = 120.0f;
     public float translationSpeed = 10.0f;  
     public float height = 0.0f;             //height from ground level 
     private Transform centre;               //transform for planet
     private float radius;                   //calculated radius from collider
     public SphereCollider planet;           //collider for planet
  
     void Start () 
     {
         //consider scale applied to planet transform (assuming uniform, just pick one)
         radius = planet.radius * planet.transform.localScale.y;
         centre = planet.transform;
         //starting position at north pole
         transform.position = centre.position + new Vector3(0,radius+height,0);
     }
     
     void Update () 
     {
         //translate based on input
         //float inputMag  = Input.GetAxis("Vertical")*translationSpeed*Time.deltaTime;
         float inputMag  = 2*translationSpeed*Time.deltaTime;
         transform.position += transform.forward * inputMag;
         //snap position to radius + height (could also use raycasts)
         Vector3 targetPosition = transform.position - centre.position;
         float ratio = (radius + height) / targetPosition.magnitude;
         targetPosition.Scale( new Vector3(ratio, ratio, ratio) );
         transform.position = targetPosition + centre.position;
         //calculate planet surface normal                       
         Vector3 surfaceNormal = transform.position - centre.position;
         surfaceNormal.Normalize();
         //GameObject's heading
         //float headingDeltaAngle = Input.GetAxis("Horizontal") * Time.deltaTime * rotationSpeed;
         float headingDeltaAngle = Random.Range(-1.0F, 1.0F) * Time.deltaTime * rotationSpeed;
         Quaternion headingDelta = Quaternion.AngleAxis(headingDeltaAngle, transform.up);
         //align with surface normal
         transform.rotation = Quaternion.FromToRotation( transform.up, surfaceNormal) * transform.rotation;
         //apply heading rotation
         transform.rotation = headingDelta * transform.rotation;
     }
 }

Why do the characters jitter at some places?