I have an Excel 2007 spreadheet (.xlsx) file with about 40 charts all in
their own sheets.

Every one of those sheets used to have their own descriptive tab name. Excel
does not seem to like that, however, as sometimes after closing, it just
deletes all of my tab names replacing them with a generic Chart3, Chart4,
Chart 5 etc.

This is highly frustrating of course. I once edited every one of those tab
names back to their original names, wasting a lot of my time in the process,
hoping it would not happen again. Alas, it sporadically happens again and
again...

Any ideas/solutions please let me know.