I've FINALLY tracked this problem down to it's symptom, as verified by myself and my users.

When new posts occur in a forum topic, my users will click on the topic and generally go to the First Unread Post.

However, if they click "MARK ALL READ", and then awhile later go to read a new post in a topic, they do not go to the first unread post as they should, and instead they start at the beginning of the thread again. 

If a user never clicks "Mark All Read" he/she never has a problem with going to the first unread post.

Again, it's only after they click "Mark All Read" that they have the problem, and only in that forum will they have the issue.

Problem will disappear again after going to a thread in which is doesn't work... going out and waiting for a new post, and going in again (without ever clicking "Mark All Read" again)

Does anyone else have this issue??  (I really hope I'm not the only one, as that means I have bigger issues!)

Can anyone please test themselves to verify?  I haven't been able to test here yet... but am waiting for some new posts in some larger threads.