A couple of questions

[MiRai]

Also something I probably should have mentioned about getting 5 instances to run at a nice frame rate. I am using ISBoxer (which I would happily recommend). I am currently running a 5 instances at the recommended resolution (which is 1600x900 - the res of my primary monitor). I tried reducing the resolution setting in ISBoxer to increase the frame rate but it made the pictures all look really blocky. I have run the game single instance at those lower resolutions before without it looking like that so I am guessing its something to do with ISBoxer.

First thing that comes to mind would be to try limiting your slaves' FPS, by default it is set to 30FPS in the background which
may be too much for your system to handle [it was for my quad core]. You can alter this setting by going into your character
set in ISBoxer, clicking on each slot, and manually changing the background FPS number. I found 20 FPS in the background
was good, 15 is great. I would not recommend going lower than 10 FPS in the background as WoW gets flakey at that point.
You can even turn the foreground FPS down to 50 if you don't mind losing out on that solid 60 FPS on your main screen.

You can also play with the CPU affinity listed in each of those slots. It is usually best to just make sure that all the cores are
selected which then lets Windows figure out which screen needs more CPU.

[Lanky]

First thing that comes to mind would be to try limiting your slaves' FPS, by default it is set to 30FPS in the background which
may be too much for your system to handle [it was for my quad core]. You can alter this setting by going into your character
set in ISBoxer, clicking on each slot, and manually changing the background FPS number. I found 20 FPS in the background
was good, 15 is great. I would not recommend going lower than 10 FPS in the background as WoW gets flakey at that point.
You can even turn the foreground FPS down to 50 if you don't mind losing out on that solid 60 FPS on your main screen.

You can also play with the CPU affinity listed in each of those slots. It is usually best to just make sure that all the cores are
selected which then lets Windows figure out which screen needs more CPU.

I updated the FPS to 50 on the foreground and 15 on the background. I'm have been playing around with the CPU affinity. I can get to a happily playable state now....but I'm thinking if I get right into multi-boxing, a fairly substantial pc upgrade might be around the corner