My users keep getting logged out randomly, WHILE using the app

[eluser]antiver[/eluser]
Users are being logged out randomly within seconds of a recent activity.

Is something wrong with my configuration?

Code:

$config['sess_cookie_name']    = 'ci_session';
$config['sess_expiration']    = 604800;
$config['sess_encrypt_cookie']    = TRUE;
$config['sess_use_database']    = TRUE;
$config['sess_table_name']    = 'sessions';
$config['sess_match_ip']    = FALSE;
$config['sess_match_useragent']    = TRUE;
$config['sess_time_to_update']     = 300;

$config['cookie_prefix']    = "";
$config['cookie_domain']    = "";
$config['cookie_path']        = "/";

And here's my DB schema:

Code:

CREATE TABLE IF NOT EXISTS `sessions` (
  `session_id` varchar(40) NOT NULL default '0',
  `ip_address` varchar(16) NOT NULL default '0',
  `user_agent` varchar(50) NOT NULL,
  `last_activity` int(10) unsigned NOT NULL default '0',
  `user_data` text NOT NULL,
  PRIMARY KEY  (`session_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

Thanks. Let me know if I need to post something else relevant.