I was thinking... since it, as far as I understand, mostly revolve around the fifth toon, could it be that the setup for this toon is different than the other 3 "adds"?

What im thinking, my setup has the main at full throttle and the rest capped at 20 fps. Maybe this have glitched on the fifth toon so you go full throttle on this one aswell. This might slow your system.

I use a Geforce 9800 GT 1GB ram, and I run 5x wow very smoothly - with the occational hickup in major cities, but I think this is because of loadtime from the harddrive's