Personally I am quite convinced that I should try procedure four on
http://word.mvps.org/Mac/DocumentCorruption.html, but I do not really know what I am looking for...
It's pretty much impossible to guess what you're looking for. The divide-and-conquer tactic is first and foremost about figuring out where the problem might lie -- and knowing where the problem is located is half the battle.
You divide the document in half and figure out which half is still having trouble. Since you're seeing problems happening in this particular document pretty much immediately after you've opened it, it seems like you'll be able to quickly narrow down where the problem might lie. Divide it in half, pick one of the halves, and start typing "the quick brown fox" over and over again. If you can do that for a couple of minutes and nothing happens, do the same thing on the other half. Once you figure out which half of the document has the problem, do it again. Wash, rinse, repeat.

Eventually, you should get to a point where you have a reasonably narrow range of the document where the problem is located, and then you can carefully comb through that section to see if you can figure out what you're seeing. This technique is just as much about figuring out where you don't have problems, because then you don't have to waste any time/energy looking at stuff that's perfectly okay.
You might also want to try posting to the
Word:Mac public forums. Lots of Word experts hang out there, and they help out folks who are running into problems. If you go through the divide-and-conquer tactic and figure out where the problem is located but can't figure out what it might be, they're likely to be able to help you figure out what's happening.
Also, if you're coming to Macworld Expo in January, swing by our booth -- 'most everyone in there is a member of the technical team, so you might be able to explain what you're seeing to one of the Word developers or testers and get some assistance that way. But that's not until January, so if I were in your shoes, I'd go through the divide-and-conquer method first and see where you get with that.
Regards,
Nadyne.