Why does Apple TV 3rd Gen keep dropping the Home Share connection after 3 to 30 minutes

I've successfully set up the Apple TV3 and it works a dream - for a short while. Then for some reason it disconnects and I have to go back ot the iMAC holding the iTunes Libarary, turn off Home Share then turn it on again and SOMETIMES the problem is corrected for a short while (anything from 3 -30 minutes) then I have to start all over again. Its really frustrating!!. The Apple TV and iTunes are running the most up to date software, there firewall on the iMac is disabled and the router is running its latest firmware. My local Apple authorized supplier can't help - all they suggested was changing the apple password, re-entering it on all devices and trying again - this hasn't work. I'll try any solution though the final one might be to take it back for a refund because it doesn't do what it was sold to do!