Formula's breaking links on their own...

Hi

I am using Excel 2003 and use a big (28mb) file which has links to 3 or 4 other files pulling a lot of data together.

The file is a Forecast file so there are 20 department tabs formatted exactly the same with the same totals consolidating in a Total tab.

I have had instances recently where on a random department several random formula or link driven cells are now their original numerical values (the links have been broken), so the formula's have been lost.

I have checked the last date the files were saved and confirm that nobody has been in and "toyed" with anything and can confirm that the links have not been broken for all links to the particular file at the time.

Has anybody out there experienced and solved this type of issue before? Or does anyone have any suggestions as to what could be causing this, limitations that could be occuring due to the size of the file or any other observations that i can take away to try and resolve this extremely annoying issue i face??

Many thanks in advance for any input you can give!