Intermittent Error: Source not found

A colleague has had the following problem since our office upgraded to
Excel 2007:

There is one main spreadsheet that contains a user-friendly menu. This
menu spreadsheet contains hyperlinks to other spreadsheets that are
usually stored in a network drive. The spreadsheets accessed from the
menu contain links (not hyperlinks - just link formulas) to other
spreadsheets on the network.

After the upgrade, when the user accesses a spreadsheet through a
hyperlink on the main menu, the links in that spreadsheet will work
correctly, but, after a number of days, they will inevitably fail with
the error Source not Found. If, however, the user forgoes the menu
spreadsheet and instead navigates to the network folder directly and
opens a file directly by double-clicking on it, the links in that file
will show as "OK" and will hold.

Any pointers to a solution would be greatly appreciated.

Many thanks,

Iain

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