Could be a slightly dodgy cable connection or a software update that needs a cold (power cycle) reboot.
This isn't meant to sound patronising, but have you tried turning everything off (TV, Sky Box, AV Receiver, Blu Ray Player, Console, ...), unplugging and then plugging back in the HDMI connectors and non-captive power leads, and then turning things back on?
If you do have multiple devices connected to your TV, try just connecting the Sky Box on its own using a different HDMI lead.
In principle the dodgy cable connection should affect all viewing all of the time, but it depends on how the firmware on your Sky Box and/or TV interact with each other over the HDMI connection depending on the mode that Sky Box is in - they could use a completely different "application" for live satellite decoding and viewing vs playing back from disk that has some particular timing requirements that are upset by an intermittent HDMI handshake / communication problem.