We've got a LG smart tv, 800 hz, so refresh should not be a problem. However every now and then the picture seems to "vibrate", it is not smooth, and this lasts a few seconds then seems to return to normal. It happens on all channels, hd or not. Obviously you only see it on moving images. Have you experienced this and what explanation is given?
What is the picture source? - does this happen with all sources (DVD/Blu-ray, Sky, etc), or just with Sky/Virgin? My first thoughts are that the problem is from the source rather than the tv if it works normally most of the time...
Exactly as Scoff says - probably your aerial. I had the same thing when watching HD channels on Freeview. Was getting ready to kick off at Samsung for selling me a duff when I realised that there's no problem playing HD DVD's nor watching HD on Netflix. I'll have to invest in a better aerial it seems if I want to watch via Freeview through the coaxial cable.