Excel 2007 rename my worksheet
 

I saved my original workbook created in 2003 to Excel 2007. This workbook
contains link and charts.

When I open the workbook in 2007. The Excel from time to time rename my
chart worksheet name

eg. I name the worksheet that contain a chart as :GroupChart, Excel change
its name Chart2 etc.

What is the problem? Now I have to keep renaming these worksheets, but
everytime I save and open the workbook again the workname is changed.