FWIW I have the same problem also.
For example, I log on in the morning and click "View New Posts" and a list of new posts appears whose length depends on when I last logged on.
Now, if I walk away from the computer for say, maybe 10 minutes and come back, I can scroll through the first page OK - everything is fine, the little yellow arrows are all there etc etc. The problem arises when I click onto the second page of unread posts. I am then greated with a list of purple folders, indicating the posts are not new anymore. The only way to jump to somewhere near the "first unread post", if to click on the yellow arrow next to the name of the most recent poster and work my way back.
The exact same thing will happen if I log on and have a huge number of unread posts, and take my time working through them. Sometimes, I will only get through say, 3 out of 12 pages before a similar thing happens and the forum decides that the remaining 9 pages have actually been read by me! I still have the list of 'unread posts' so to speak - so all is not lost. It is just a nuisance to try and get to that elusive 'first unread post'
, particularly in some of the longer threads.
Is this what you are describing elnero??