ATV recurring audio issues - distortion and echo over HDMI every hour

We were really psyched to get an Apple TV for Christmas. We plugged it in and it played and operated beautifully.

It was very disappointing to encounter an audio issue after an hour of use where the sound from anything coming through the ATV (purchased movie, rented movie (iTunes), Airplay music or video from iPad and iPhone and PC) degrades into a mess of crackly distortion and echo. It happens after every hour of use. Our TV has 4x built-in HDMI inputs also supporting connections from our Virgin HD Tivo, our Toshiba Bluray player and my Alienware laptop. The sound from these other devices connected via HDMI to the same TV is just fine when this happens. It's not the TV.

The 'fix' to this point has been either reload the ATV or to forwardwind/rewind the movie and muck about in the menus (although not sure how effective any of that has been). As you can imagine, having to do this every hour is a bit of a buzz killer, while watching the latest 2+ hour long blockbuster via iTunes.

I have changed the HDMI cable to no effect.

I have changed the input interface on the TV to no effect.

I have systematically changed each of the audio settings in the ATV to no effect (16 bit sound, sound check, etc)

I have also mucked about with the audio settings of the TV to no effect.

I am not alone in experiencing this. Googling brings up a couple of hits of folk suffering the same issues with their ATV - even the same time intervals - but I can't find a post where someone has said 'this fixed it for me'.