Access losing links

I have an Access application with links to SQL Server tables. Every time I start the application I get error messages indicating inability to read the linked tables. When I have relinked everything works fine, but eventually I log off the application and get the same problem the next time I open the application. This has happened in Access 97, and in Access 2000 after conversion. It is not happening in my other Access applications, several of which link to SQL Server.