XLTM file corruption

I have a template with embedded macros that I created using Excel 2003. I
used the convert function of 2007 to convert it to a 2007 template (XLTM).
That portion works fine, I can save and reopen it without issue. I can
reopen it, and make changes to the charts I have embedded and the macros.
After I save the changes and exit, then try to use the template or reopen it,
it frequently gives me the following error message, "Excel cannot open the
file 'name.xltm' because the file format or file extension is not valid.
Verify that the file has not been corrupted and that the file extension
matches the format of the file."

Any suggestions on how to keep the file from being corrupt? Should I not
convert it? Fortunately I had a backup, so I didn't really lose anything but
time.