checkout payment page looses session?

[infinifire]

Ok, I have a heavily modify cart install that basically on rare occurences puts the customer in an endless loop of redirects due to billto variable not being set on checkout process page. I've added logging into this page and dumped the users session that hit that area. All of these sessions are missing in the navigation history any reference to the checkout_payment.php page. Meaning that every hit they make to that page is either not in any session???Impossible or it would redirect out again or isnt being saved inot the session for some reason? HELP HELP HELP