I am getting Airplay dropout from my iMac (El Cap - 10.11.5) to both my Apple TV and my Airport enabled Yamaha receiver. Problem only seems to have started since last El Cap update. All works fine to the ATV and Yamaha from both iPad and iPhone. and from my MacBook Air (also with 10.11.5!) Problem seems to be unique to the iMac. Have reset the router, no solution. All ideas gratefully received...