Hdd recording sky channels

hi I have a goodmans recordable hard drive dvd, the problem I have is when i record any sky channels ie, sky sports or sky news or any films the picture jump or pixelates, but goes normal recording when adverts of same programme come on, also records normally when recording programmes of other channels ie disney, itv, eurosport etc. only effects sky produced recordings, its a virgin set top box, please can anyone help?