At my mum's house my sister has sky television in her room ( where sky was first installed )
My mum recently had the service connected to the tv in her room
However , the picture on one particular channel keeps breaking up on my mum's tv ( all other channels are ok ) -but the picture on the same channel is perfectly ok on my sister's tv
what is the connection from the sky box to your mums tv? does the tv also use this as the feed for the terrestrial channels?
if your mum has a VCR or DVD recorder attached to the TV this could be interferring with the signal as the Sky box and VCR/DVD could be 'transmitting' to the TV at a very similar channel.