Here's a example of the problem on your own site:
Go to this page (/reviews/showcat.php) where you'll see there's seven products. Click on the first product. Here you get a 'Previous Product' and a 'Next Product'. The 'Previous Product' links that very same product.
Clicking on 'Next Product' takes you to the seventh product that was previously shown on /reviews/showcat.php, clicking on the 'Next Product' link there then takes you to the fith product that was previously shown on /reviews/showcat.php
I'm also wondering if this has anything to do with the fact that there doesn't seem to be a standard number of 'show product preview' thumbnails being shown. On your site there's sometimes only four product thumbnails being shown... sometimes five and sometimes seven.