Is there a software/firmware update you can apply?  Has one recently been applied that you roll back? Have you tried resetting to factory settings?  Does this happen with all inputs or just one specific input?  Tried a different HDMI cable?  If it's happening on Sky but nothing else are you sure it's the TV and not the decoder that's at fault?

I'd try those things first and if it's still a problem it probably needs to go into the shop.

I think there is a picture test option on most LGs in the menu , you may want to try it so that you can eliminate inputs and cables . I would be interested to see how you get on with LG if it is a panel fault being only 3 years old .