1080p sound issues

I have purchased an Apple TV (3rd Generation) and it works brilliantly, but only in 720p. When I try to stream 1080p movies from my iTunes or watch them on Netflix, the sound briefly cuts out for a second every few minutes.I don't know whether it is the tv's fault but I watch blu rays on it all the time and no issue there. I have also tried it with a high-speed HDMI but no improvement. Does anyone know what I can do to resolve this issue? Thankyou