This may or may not help, but is the one client that is lockup up running questhelper? There are some known unresolved bugs where questhelper can crash clients. Once I hit 70 on my shamans and got rid of all my quests and started doing just dailies I could no longer run questhelper on my shaman team without crashing. Works just fine on my leveling teams.

If you are not running questhelper try disabling all your addons and playing for a bit. It could very well be because of some addon that is out of date and causing issues.