Problems Using Outlook in Cached Exchange Mode

Running Windows SBS2003 w/ Exchange 2003 with dual NIC's (one outward and one in).  One of the client PC's (running XP SP2) is used by the owner of the business and she monitors all users mailboxes in Outlook in addition to her own.  Her Outlook is set to use cached exchange mode as a way of ensuring that she will have a local copy of everything in Outlook in the unlikely event of an Exchange failure.  Every few weeks, she encounters an error message saying that Outlook is unable to access other users folders and she can no longer access the other users mailboxes.  The error is cleared and the mailbox views are restored by simply toggling the cached exchange mode and restarting Outlook.  When we do this, she also gets a flood of emails which have obviously been held up somwhere in the system. During this period however she does receive many (but obviously not all) of her emails without any apparent problem My suspicion is that these two problems are both related to use of cached exchange mode.

Can someone shed some light on what is likely to be causing these 2 problems and how best to get these cleared up.

Thx,

Bill