Had a problem with my E4200 for a while, where if I attempted to use Airplay over 5Ghz Wi-Fi, the whole network would “reboot/bounce”. Finally came across a solution posted by canadamg over at the Apple discussion forums.
In router management, apply these settings:
Applications & Gaming -> QoS -> WMM -> ENABLE IT
Administration -> Management -> CTF -> DISABLE IT
I can confirm this works on my router with firmware version 1.0.03.