Word 2010 undo feature is greyed out / broken

Hi,
I've met this problem three times in the last few days: Word 2010 undo feature is greyed out / broken
I'm running Office Professional Plus 2010.

I'm working on a long document 390 pages (my dissertation
When I started out, the undo feature worked perfectly and I could undo dozens of steps.
But after saving the document with a new name, the undo feature "breaks". It is greyed out and I cannot use it.

If I run the re-install file and select "repair" for WORD, then everything goes back to normal.
But, after I work on the document for a while, and begin saving it with a new file name, the same thing happens.

I've searched on-line, but cannot find any information about "undo" breaking like this in Word 2010.
What is causing it and how can I prevent it?
Is the document template that I'm using somehow defective? Is it repairable?

Thanks very much for any advice!