Carl, if I remember right, you had some other weird problem with the board... I'd have to search through the old posts to find it. I wonder if there is a problem with the way your computer or browser handles cookies?
Edit: OK, I went back and looked, it's actually the same old problem
I was wrong about cookies. Basically, the board only bases "new posts" on the last time you visited. It even works across computers--I can check new posts at work, come home, and check again, and it remembers the correct "last visit" time. Thus this last visit time is stored on the board here and not on your computer.
Looking at the links Colin posted, it looks like the main reason the "last visit" time gets messed up is that if you close and reopen your browser, that "last visit" time resets. I suspect any other action that essentially does a re-login (even if you don't notice it) will cause the same issue. I'll keep trying to find the cause of why this feature isn't working for you...