Hi , my pioneer vsx ax5i has start to play up. when using it the sound goes, this can happen after just 10 minutes , across all in puts. i turn it off at the plug, when i switch in on again the next day the sound is back. i run kef 2005 speakers,dvd757ai via i link,pioneer tv via optical lead,humax freeview via optical. it went to my local store for repair but they found nothing wrong.has any one got any ideas?