Microsoft Excel Charts?

I have excel files (version 97-2003). I recently upgraded to office 2007 and now have a sporadic problem with the charts in my files. Sometimes but not all of the time, the format of the x axis will change. Instead of the file opening with the format set at vertical (the way it was saved), the file will open with the format set at stacked. Sometimes, even after correcting it to vertical and re-saving the file, I can immediately re-open it and the format will have changed back to stacked. It doesn't happen all of the time but it is really frustrating as I have to constantly review each graph prior to printing to ensure that the x axis format is correct. Help!!!