I was having some problems and I noticed that the Battle Net Update Agent had several copies running. I did some google-fu and people have been complaining about this destroying game performance and eventually their entire machine. A couple times my game has felt lagging and when I checked there were 5 processes running. I killed them in task manager and everything was back to normal. Look for a process called Agent.exe. Don't kill them all, as the parent will just restart in a minute. You want to kill the ones with no command line listed in task manager when you turn that column on in the View menu. Combined those five processes were using about 900 MB and it kept growing. That's a big deal on my machine with a meager 14 GB. I've also seen people complain that it uses a lot of network resources as well.

Oh, avoid wowhead.com when you can. I've seen those pages take from 800 MB to 1.8 GB (more than an instance of WoW!) on my machine.

Let me know if these things help and if not we can try some other stuff. I'll keep my fingers crossed.

- Souca