Session expiring before expiration period.

My sessions seem to be expiring before their expiration period. I've researched this quite a lot and am unable to resolve it. I thought may it was down to Asynchronous ajax calls (session race condition) so I extended the session class via MY_Sesssion.php to only regenerate the session if it's not an AJAX call. 

I've included both that function and my config below. I'm using CI version 3.1.7

PHP Code:

class MY_Session extends CI_Session {

    // --------------------------------------------------------------------

    /**
     *
     * Do not update an existing session on ajax or xajax calls
     *
     * @access    public
     * @return    void
     */
    public function sess_regenerate() {
        $CI = get_instance();

        if ( ! $CI->input->is_ajax_request())
        {
            parent::sess_regenerate();
        }
    }
} 


Config:

PHP Code:

$config['sess_expiration']        = 7200; //= 2 hours
$config['sess_driver']                  = 'database';
$config['sess_save_path']        = 'ci_session';
$config['sess_table_name']        = 'ci_session';
$config['sess_match_ip']        = FALSE;
$config['sess_time_to_update']          = 300; // 300 = 5 minutes
$config['sess_regenerate_destroy']      = TRUE;