Since I'm toying with the TinySlideshow tool myself I thought I'd try and be helpful.
If you take one of the list item elements within the 'slideshow' ul element and paste it multiple times do you have the same problem? By that I mean take an element you're sure is working and repeat it 10 or so times and see what happens. I did and it worked okay. Depending on your browser you may want to check to see if you're getting any javascript errors elsewhere in your code to see if anything is interferring with this code.
Example element to repeat within ul.
<li>
<h3>Blue Fish</h3>
<span>photos/blue-fish.jpg</span>
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam ut urna. Mauris nulla. Donec nec mauris. Proin nulla dolor, bibendum et, dapibus in, euismod ut, felis.</p>
<img src="thumbnails/blue-fish-thumb.jpg" alt="Blue Fish" />
</li>
Hope this helps.