Have you read this thread?

If you're using multiple video cards - especially if you're using one card per monitor, you need to set the "home" monitor for each instance in game. WoW does NOT like it if you start it on one monitor and then run it from another, you will get HUGE performance loss. So set the main to "Primary Monitor" or "Monitor 1" and the slave to "Secondary Monitor".

Try swapping to DX9.

Try disabling vSync.

Make sure you have disabled all out of date addons. Personally, I'd disable all addons period and enable them a few at a time to make sure they aren't the problem.

Delete your current setup and rerun the setup Wizard for ISBoxer.