Still can't figure it out.

Again, sometimes when I alt-tab or hit the windows key, both instances of WoW will become extremely choppy (like <5 fps) instead of the usual 60+fps. The choppiness will go away eventually or will immediately cease if I close one of the WoW instances. When it is choppy, the CPU usage of each WoW instance is much LOWER than it usually is, and overall CPU usage on my comp is only <20%.

Would really love to find a solution to this since it really limits my multi-boxing ability...