I'd been wondering why it was behaving oddly. ^^; Now I shall get to go read up on phpBB (which I needed to do anyway).
Fwiw, I like this look. It's very clean and functional, if not very decorative (which I can do without) and rather bright (which I can deal with).
A little background: To integrate the forum, I was dynamically rendering a "wrapper" for the forum template based on the main site's current Wordpress theme. I had this set up to render once on first visit, then cache the HTML in the database for future reference. It worked fine, for the most part, but it's more load-intensive than I'd like (since it has to wait for the forum software to render its templates, then apply the wrapper based on our cached markup). When you have several active visitors, it can slow things down - especially since the server hosting our site isn't exactly the speediest machine on the planet. The "white page" effect people were seeing was the fallback condition triggered when Apache's session timeout was in danger of being reached - dump the already-finished forum HTML along with the unformatted links and text from the wrapper.
When I get some free time, I'm just going to modify the forum theme to add a static wrapper to get the look and functionality I want and call it a day. The only disadvantage: it won't automatically update the menubar if we add or remove items.