I had an engineer come out and he said it was normal for the sound to cut out when changing channel etc. When I told him it was something that had just started to happen and other people were having the same problem he said the only option would be to change the V6 box.
I decided to keep the box I have for now as there were recordings on there that I needed to keep and my data could not be transferred to a new box.
After the engineer left I turned everything off for 15 minutes and after the reboot the sound has been fine and I have had no further issues. Not sure if you have tried a reboot but thought I would let you know it has worked for me so far.