If I was you I would back up your version of category_product_listing.php.

Then I would hover over a 'problem' link on the page and note down what address the link is pointing at.

Then I would install the default version fo category_product_listing.php. Check to see if the problem still exists. If it does then you can compare the URL that the first page was pointing at to the URL the default version points at.

I suspect that you are going to find that this is not to do with IH but lets wait and see.