Mine does the same, but I find it happens during bad weather as I live on the coast and the wind and rain can be quite heavy/strong here.
Sky recommends trying to restart your sky box by switiching of by the remote then by the plug wait 5 mins and then plug it in again.
I am not sure if this will work on an older model like yours?
I try cleaning my sky viewing card on my clothes on even with a rubber/pencil eraser.
Also of course check all the connections, make sure that all the wires are pushed in properly, and that weather has not eroded your sky dish or cable.