I have had similar behavior from my rig pretty much since the beginner, and through multiple upgrades. I 5-box, on a quad-core system with 8 gigs ram and running 64-bit XP pro.

The CPU peg problem can happen any time one of my WoWs is entering the world or an instance. Bringing up the task manager and then closing it, which forces a full window redraw. fixes it.

I have just been living with it, frankly.