Does it sit somewhere where it's getting hot, like in an enclosed space under the TV? If it sits with something else on top, like a DVD or video player, then you might want to swap them over to allow it to breath a bit more.
The other thing to do is to give it a hoover and suck all the dust out of it. I'm pretty sure they have a fan inside, so it may be that the fan is clogged with dust.
If not then it may well be your hard-disk on the way out. It's not too difficult to change them (although you lose your recorded programs) but take the opportunity to fit a much bigger drive for about £40.
http://www.morlocks.co.uk/sky.htm