Chart worksheet names keep resetting

Hello,
I have been working with 3.5 - 6.5 Mb excel 2007 files that contain one
worksheet of data and up to 20 individual chart worksheets. I have each sheet
named for reference, but every so often when I open a document, I find all of
the chart worksheet names have been reset to Chart 1, Chart 2 etc. and there
is nothing I can do but re-rename. This has happend multiple times. The one
data worksheet always keeps its assigned name.

Is there a reason/fix for this? I would like to stop this from happening.