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These symptoms are ones that I am familiar with, and they point to a premature fault occurring in the set's Mainboard (this board has the processor that controls the turn on/off commands). This is NOT within normal operation of the set- and it's not a setting/adjustment issue. The only thing on a user level you can try is to unplug the TV, then press and hold the power button on the TV itself for 20 seconds and release, wait a minute then plug the set back in and power it up to see if the issue persists.
Beyond that you're looking at a hardware issue, and more specifically- the Mainboard needs to be replaced under warranty. In that case you would need to contact LG and setup a warranty repair ticket on the set:
Let me know if you need further assistance.
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