I've been experiencing this for a number of weeks. I initially put it down to a single channel issue on a particular evening, but it's happened now on multiple channels, including just now on sky one hd.
Usually it is just a sound stutter, but occasionally the picture will hitch at the same time.
On both live TV and recorded content the sound will stutter approximately every 15-20 minutes and it isn't channel specific. I've seen it across BBC and Sky channels. I can't confirm on demand as I don't use those services via VM.
(Not sure this needed its own thread - I was simply echoing the same issue as reported in two other threads in the TiVo and V6 forums.)