Are you running in "MTH" or "HOME" mode on the WIU? This could be being caused by your iphone especially when running in in MTH mode. Your phone quickly discovers that the Wi-Fi connection your phone is currently connected to does not have internet access and it starts scanning for other networks that can provide internet access. Meanwhile, you phone will intermittently drop the connection to your wireless your MTH wireless network and so begins the problem you described earlier.
I troubleshoot this kind of problem all of the time with Apple devices. one thing you can try, toggle OFF the option in the Wi-Fi settings "Ask to Join Networks". Turning this off has served me well when users attempt to use Hotel Wi-Fithat require web authentication before internet access is granted.
