Fix bug in Magento webshop

I have a problem with magento, or a extention of it. The problem is that somewhere in the code there is a loop. The php thread thats assigned to a session fills up with memory and 100% cpu. Then when the buffer is full, the user sees a memory exhausted error. if they refresh, they get the same error as the session id is still linked to their cookie. When removing the cookies, it works. But it can take 5 mins, but the error will return again. The error most occurs if you go to the checkout, then return to the normal homepage (https to http) Only bid on this project if you are a linux / magento wizard, no cure, no pay! This is a live website, i will make a backup before you begin but realize there are people visiting the site!