Windows randomly freeze

I've been leveling 5 characters on Lordaeron with ISBoxer with no problem at all, but I decided to try Frostwolf for a bit. So I started leveling 5 characters there, but for some reason one or more of my WoW windows freezes every 20-30 mins or so. I then have to end the process in task manager and load up my character set again.

The freezes seem like they're completely random. I freeze when I'm simply walking along the path, as well as when I'm fighting in a dungeon. It's getting quite annoying now and I've never encountered this problem before.

Does anyone know why this might be happening and how I could fix it?

- Thanks.