It only fails when you try to view a forum section like the Social Lounge...
My £££ (not that anyone in the UK has any left) is on a fault in the database when it's being queried by the PHP script to populate the list of threads. For some reason, this list is a load of corrupt balls and the PHP script isn't executing due to being fed with duff data, which means a white screen because you're not getting any HTML back.
Very rough, uneducated guess after a few drinks but hey, this is my hypothesis. When I've had some sleep I'll be able to put my mind a bit more to it, but David should check his Event Viewer on his server (which I know is running Server 2003, so it shouldn't be hard to figure out the issue) and take it from there. Look for events at, or around, the time of the white screens. Right now, I've got "Revolution 9" by The Beatles stuck in my head (great track, plus it has clips which are apparently from the unreleased track "Carnival of Light" so that kills two birds with one stone), and I live down the road from a historic murder scene, so all this mystique and uncertainty is freaking moi out and making moi feel all warm and fuzzy.
I are bed.