Spontaneous format changes in Excel

I have a user who says that he experiences spontaneous format changes in MS
Excel when he is working in his Monthly Receivables Analysis 2009 workbook.
Every month beginning with the start of the fiscal year, he adds more data
into the workbook so the size of the workbook grows continually until the end
of the fiscal year, when he starts a brand new workbook. Currently, the file
is 82,087KB in size (a very large spreadsheet). The user says that when
working in this Monthly Receivables Analysis workbook, if he changes the
decimal places of a cell range on the summary sheet from two decimal places
to none, and then saves the file, the cell range switches back to the
original two decimal format. In addition, sometimes when he changes the
number format category of a cell range from Accounting format to Number and
saves the workbook, when he opens it later the number format category
switches back to Accounting. He says that the problem is intermittent, and
that he does not experience these issues when working in smaller Excel
workbooks. Have any of you experienced this phenomenon, particularly when
working with very large workbooks? Any insights you could provide would be
appreciated.

-- "There is no knowledge that is not power." -- Ralph Waldo Emerson