Philips bottom part of screen darkens after a few seconds of use?

Philips bottom part of screen darkens after a few seconds of use?

My philips tv model is 32PFL4507/F7. It has worked well since I bought it but recently the bottom section of the screen has darkened. This only occurs after the tv has been turned on for awhile, sometimes seconds. It is sometimes nice and bright but goes dark eventually.

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