Hi Chris. Welcome to justanswer.
How long have you had the set for and when did the problem occur?What is connected to the TV for devices (DVD, cable box, etc.), and do you have the same issue regardless of which Device you're watching?
If you bring up the MENU on the TV, is the MENU itself also affected by this problem?
The MOST common cause of the problem you're having is a faulty T-Con. Power supply is not outputting the rated voltage to the T-Con. The board will cost around £40-45. You will need to call a local technician to change the board.
This is a very common problem and is totally repairable.
Let me know if you have any other query.
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You are welcome Chris. Have a wonderful evening and thanks for the positive ratings.