Crappy old Thomson box? If so, may well also be dodgy capacitors in the PSU, as well as (or instead of) a failing HDD.
Get it replaced by Sky, for free, with a nice new DRX890, which will have the benefit of being compatible with Sky On Demand and any other new features.
http://www.skyuser.co.uk/forum/sky-h...ox-scheme.html
Sky's run this offer a few times. Mrs D and I had our old Thomson box replaced for free last year.