That WD97 did falter on large documents a lot but your comment about no update has me read http://support.microsoft.com/kb/171159 about fixes in the update for map issues. There are likely more there as well in later patches but I decline to find them all.

There's also the old issue of the normal.dot collecting cruft over time and causing odd failures. If you are unwilling to backup normal.dot then try another user account with the file in that user's account and local folder.

Good luck,
Bob