I suspect it's not your tv, but the transmission getting interrupted. Never had satellite, but cable often does this and/or freezes for a minute (especially since virgin took over from telewest) so I assume satellite digital transmission is similarly subject to interruptions eg when the floaty robot gets a bit of space fluff on it.
The most usual cause is mains electrical interference. Something as simple as the thermostat on your freezer cutting in (or a neighbour turning a light switch on or off) can cause a TV picture to break up.
Plugging in your TV (and any associated equipment, such as home cinema sound systems) via a 'mains conditioner' (which provides surge protection and rejects mains interference) will probably fix the problem:
http://www.maplin.co....gle&U=219077&T=Module
Thank you all for your answers. It could probably be any of these reasons but I can't understand why it's happenening now. I have had Sky for quite a few years so I am used to it happening when the weather is bad, but this seems to occur at random times.