I've only had it happen once so far, but on exiting Galv's room in AV, I had the fps on my main screen drop down rapidly, all the way down to 0 fps, and then stay there while my slaves got chewed up by the alliance. To fix it, I closed one of the WoW.exe processes from task manager, then started it again from the Keyclone command. Everything worked find from there after I logged back in, but I'm still confused about what prompted it. I didn't have any totems down, and there wasn't combat or AoE effects going on around me.