Portion of webpage is blank

I am hoping someone can point me in the right direction. I have several categories and sub categories on my webpage. When I am within a sub category, there are multiple products that result in multiple pages to view to see all the products. The first few pages that I view shows all the banners, sideboxes and pictures appropriately. But then for some reason when I get to page 4 (for instance) all the banners, sideboxes, formatting is on the webpage are fine, but the product images do not show up and the section that allows you to advance to the next page or to go back to the previous page is no longer available.

The links all seem to be set up. I don't know where else to look for the issue. Please help!

I have included two links to show the issue

The entire page loads correctly - http://megasportsmall.com/index.php?main_page=index&cPath=10_55&sort=20a&page=5

When I select page 6 of this category the page does not load correctly - http://megasportsmall.com/index.php?main_page=index&cPath=10_55&sort=20a&page=6

1. press reply and fill in the posting tips that appear above the text entry box

2. Are there debug messages appearing in the /cache folder of zencart
   eg here megasportsmall.com/cache
   You need to check the folder using your file manager or ftp client

3. Have you checked your server logs for errors around the time you do this

4. what add-ons are there etc

and I should add this

https://www.zen-cart.com/tutorials/index.php?article=82

probably the most visited page in the tutorials

I am running version 1.3.9h and it was a fresh install in November 2011. The instructions from Zen Cart were used for the install - upload via FTP and run zc_install. I am using Filezilla for FTP.
The addons installed are in the order totals modules - discount coupons, group discount, gift certs, shipping, subtotal tax and total. In Payments - Paypal Express Checkout only. In shipping - UPS only.

I am not sure when the problem started. I have read every article I could find on this issue, including article 82. The last error log in my cache - myDEBUG occurred on Febuary 5, 2012. I have been able to reproduce the error several times by going to different web pages on the site and no new error logs have been generated.

I have tried to remove the entire category, subcategory, products and images from the site and reload using ftp to ensure that it was not a load issue. This has not helped.

Any assistance you can provide would be appreciated.

go through turning off products that would display on that page until you find the bad one