It happened to me when I first got my setup quite often.
Basically the things I changed are as follows:

-Make two copies of WoW (and only two). One for your main and one for the slaves.
-Turn the graphics as low as they will go on the slaves.
-Set your max fps on the slaves to 10 or 15. /console maxfps 10
-Run your slaves on two processor cores and your main on two processor cores.
-Having as few mods enabled on the slaves as possible always helps.