Can anybody help please

Over the last couple of weeks my sky + picture keeps getting pixels on the screen, freezing for a couple of seconds and then going ok for a couple of minutes then it does it again. This is the strange bit it happens on all of the terrestrial channels IE: BBC1,BBC2 it also does it on some of the sky channels but not all of them. I just have the normal setup the SKY+ pace box

any ideas anybody

Thanks
Nik