Slideshow - Next / Previous Buttons Skipping Slides

I've been working on a Javascript slideshow and have it working except for one problem that when the "next" or "previous" button is clicked for the first time, it skips the following slide. As a bit of a background, my client wants the slideshow to load a random slide when the window is opened, and also play the slideshow in a random order. However, when the "next" or "previous" button is clicked, it is meant to go to the correct sequential slide. For example, if it randomly loaded on slide 5 it should go to slide 6 if "next" is clicked and slide 4 if "previous" is clicked.

If nothing is clicked it will just go to another random slide. Once a button is clicked the client wants the slideshow to stop. I have all of this working, except when the "next" or "previous" buttons are first clicked, one slide is skipped. After clicking the buttons for a second time, it works fine. For example, if it starts on 5 and the "next" button is clicked it goes to 7 instead of 6. However, if it is clicked again, it works fine. The same is true for the "previous" button, altough it does not go to the incorrect slide but takes two clicks to go to the previous, correct slide. You can see what I'm talking about here: [URL]

My code is below.
<script>
<!--
var SlideShowSpeed = 6000;
var CrossFadeDuration = 20;
var Picture = new Array();
var Description= new Array();
var Header= new Array(); .....