Massive lag that only occurs at certain times

[Raiden333]

This is kind of a weird bug. The game is running really well at medium settings on my PC with no lag at all, but if I open my inventory, open my journal, grab a note off a table, or hit escape, the game freezes and the audio loops very annoyingly for about 10 seconds before it opens.

What makes it weirder is that if I open inventory/notes/main menu shortly after I've opened it before, this doesn't happen at all. It's like it only happens when I open a screen after at least 30 seconds of gameplay, which makes it happen 100% of the time.

edit: If it matters, I'm on Vista 32bit.