CodeIgniter v3.1.11 - odd session behaviour

Hello,

I've created a shop application which very rarely shows some odd behaviour in the checkout that I am hoping someone will be able to shed some light on.

Just as any shop site, mine has a membership system and a checkout. Very rarely, logged in members see another member's delivery address in the checkout; seems to be details of a random member that are pulled.

I know for a fact that these members aren't doing anything strange with their session cookie (which is encrypted) nor are they randomly logged out. They simply just see another random member's data in the checkout.

The site is using CI's built session library and doesn't have any other issues with sessions. I can't see any pattern to this bug, so my debugging efforts have been pretty fruitless. Any insight into this will be much appreciated.

CodeIgniter v3.1.11
Here are my session settings:

PHP Code:

$config['sess_driver'] = 'database';
$config['sess_table_name'] = 'session_data';
$config['sess_cookie_name'] = 'mainsite';
$config['sess_expiration'] = 2592000;
$config['sess_save_path'] = null;
$config['sess_match_ip'] = false;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = false;