Zen Cart adding incorrect products into shopping cart

Hi, I have a zen cart site and for a few months i've gotten more and more complaints that when they select product A it is adding product B. I've done some testing and find that it works perfectly on the product list page but not on the product info page. So on the page where it list a lot of products if i click add to cart it work great...but when i click into the product and click add to cart then it adds the wrong product. Hope this helps. We need a quick solution and let us know what files were changed so we can back up. Thanks.