Bug Trouble: Spider deviating from parabola path

Hey everyone,

In the middle of tidying up the last few bugs (that we're aware of :P) in our game before sending it off for review.

I've got one here that is giving me problems simply because I can't reproduce it.

The core idea is I've got a spider game object that moves in a pendulum-like fashion (parabola curve.) The object has a simple box collider for hit-detection and moves back and forth between two set points. Occasionally, one of the spiders will deviate from the course and move straight up along the Y-axis. I'm at a loss as to what could cause this. Any ideas?

using UnityEngine; using System.Collections;

public class swingSpider : MonoBehaviour {

 public Transform start;
 public Transform end;
 
 public bool startToFinish;
 public bool finishToStart;
 
 public float slerpStartTime;
 public float speed;
 public float height;
 public float verticalVelocity;
 public float swingLength;
 public float swingPower;
 public float distance;
 //public float swingTime;
 public float curTime;
 public float waitTime;
 public float time;
 
 GameObject spider;
 //public GameObject A;
 //public GameObject B;
 
 Vector3 spiderPos;
 Vector3 startPos;
 Vector3 endPos;
 
 // Use this for initialization
 void Start () 
 {
     startPos = start.transform.position;
     endPos = end.transform.position;
 
     startToFinish = true;
     
     slerpStartTime = Time.time;
     transform.position = start.position;
 }
 
 // Update is called once per frame
 void Update () 
 {
     
     if(startToFinish == true)
     {
         time -= Time.deltaTime;
         if(time <= 0)
         {
             distance = Vector3.Distance(transform.position, endPos);
         
             if(distance > 0.1f)
             {
                 height += verticalVelocity * Time.deltaTime;  
                 verticalVelocity = Mathf.Lerp(swingPower, -swingPower, (Time.time - slerpStartTime) * speed);                            
                 Vector3 basePos = Vector3.Lerp(startPos, endPos, ((Time.time - slerpStartTime) * speed));
                 Vector3 resultantPos = basePos + (Vector3.down * height);
                 transform.position = resultantPos;
             }
             if(distance <= 0.1f)
             {
                 ResetA();
             }
         }
     }
     
     if (finishToStart == true)
     {
         time -= Time.deltaTime;
         if(time <= 0)
         {    
             distance = Vector3.Distance(transform.position, startPos);
         
             if(distance > 0.1f)
             {
                 height += verticalVelocity * Time.deltaTime;  
                 verticalVelocity = Mathf.Lerp(swingPower, -swingPower, (Time.time - slerpStartTime) * speed);                            
                 Vector3 basePos = Vector3.Lerp(endPos, startPos, ((Time.time - slerpStartTime) * speed));
                 Vector3 resultantPos = basePos + (Vector3.down * height);
                 transform.position = resultantPos;
             }
             if(distance <= 0.1f)
             {
                 ResetB();
             }
         }
     }
 }
 
 void ResetA()
 {
     if(startToFinish == true)
     {
         startToFinish = false;
         time = waitTime;
         slerpStartTime = Time.time + waitTime;
         transform.position = end.position;
         distance = 0; 
         finishToStart = true;        
     } 
 }
 
 void ResetB()
 {
     if(finishToStart == true)
     {
         finishToStart = false;
         time = waitTime;
         slerpStartTime = Time.time + waitTime;
         transform.position = start.position;
         distance = 0;
         startToFinish = true;
     }
 }

}