Might be a stupid question, but are you telling ISBoxer to use different CPU Cores for each instance of WoW?

That was the issue I had a long time ago.

Also, what version of Windows are you using?

Also, also, you don't need to run your CPU at such high frequencies (at least for WoW), I have the same CPU and run 6/7 instances at the same time across 3 screens with no issues and max fps.