Sharepoint users are logged out

Everyd开发者_开发知识库ay at sometime (mostly near the noon) all users of our sharepoint (2007) except "System Account" automatically get logged out and cannot login for about 30mins. I have no idea what possibly causes this situation. Can anyone help?

Thanks in advance.

Update: we use Active Directory for user authentication. At situations described above we can successfully login to Active Directory but it seems that SharePoint fails to login.

When you say 'cannot login for about 30mins' do you mean that the site is inaccessible or users get an Access Denied page (and I mean the SharePoint one)? If the site is inaccessible then the SharePoint web server is probably down.

On the 30 minutes what are the events in the Windows Logs of the server ?

What can be the cause : Maybe the primary DNS is filled in, and not the secondary. And maybe there is a reboot of the first DNS server ? Or maybe a network firewall or proxy is acting weird ?