I have that exact hub, and am having all kinds of stability problems (miners won't run for more than six hours without going zombie). See the main 2pac support thread for details. Don't know if it is related to the hub, but if I were starting over again, I would be looking at something else.
I Think this usually means that in sometime there was not enough power to run them, so they went zombies. Try lowering frequency in 10 or 20 mhz, and lower just a little bit the VCore, and see what´s happen.
Actually you made me think that in my house, I have a really unstable power source, it usually loose power in a flash moment, these kind of thing would make the sticks goes into zombie mode!