Word crashes during Find

[BPB]

When I use the Find command in one particular document, Word freezes, and I
have to force it to close. [more details below]
Any suggestions?

I first encountered this problem this morning. It only seemed to freeze when
I used two or more words in the Find field, and even then it worked
sometimes. Now this afternoon, single words and numbers have the same effect,
every time.
A colleague had this problem with this document a couple of weeks ago. But
I've had no problems before or since then, till today. I use it, edit it,
save it, close it, post the latest version to a doc repository, then re-open
it and continue to use it many times a day, with no issues until now. No
major new edits have been made since I closed it down yesterday, and I'm the
only one who's been using it for several days. (Even after posting it to the
repository, I immediately check it back out for myself.)
I'm using Word 2007, but I 've been saving this 350+ page document as a Word
97-2003 doc for months (without any issues) for non-2007 users in my office.
Thanks a lot for any help you!