Docs can't be read after server is idle

I have several databases whose documents contain multi-value Reader fields. The Reader fields contain the name of one Group plus two roles. Users are members of the Groups.

Problem: After the server is idle for a time (say, overnight) the users can open a database but see NO documents. After the administrator restarts the Domino service, the users can see the documents.

Users who are assigned one of the roles in the Reader field can always see the documents. The server is also assigned one of the roles.

Users can always use Notes mail and see Person and Group documents in the Domino Directory.

Windows NT Server and XP clients run 6.5.1.

Can anyone explain how to fix the problem?