Does it happen using Moodle NOT using AICC content? For example, if you open a page for editing and leave it for (say) an hour and then save. What happens?
Are you *sure* that the session timeout is set correctly? Is this being reported properly on the settings page (even if you've set it in config.php)?
EDIT:
The session timeout depends on cookies, so you should be able to check the Moodle cookies in your browser. There should be two (IIRC) - one for remembering the username and the other for the Moodle session. The lifetime is set here. So, it's possible that a browser plugin or some firewall madness could be behind this.