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.
This is weird. I have a 230+ page doc with a doc map and use the scroll box on it a lot to go to different spots on it. All of a sudden (and nothing changed that I know of except it gets longer all the time) when I move the scroll box it pops up a yellow box with the page # and desc in it where I am. I do like it and was wishing I could know where I was scrolling in the map. Wonder what triggered it now? I've been using this doc map like for a year. And it's only Word97 so it hasn't been updated.