571597 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. It seems really weird when it's doing this as it doesn't happen everytime but it's definitely happening. The 2nd zen cart site has a lot of order that get duplicated and many times we do not get the extra credit card information so we then have to call the customer to get all the information. So basically a lot of orders get duplicated and many time we don't recevie the extra email with CC info. This cart does not charge the card so we can make many test until it's working. We need a quick solution and let us know what files were changed so we can back up. Thanks.