How long have you had the set for, and when did the problem occur?
Additionally, what is connected to the TV for devices (DVD, cable box, etc.), and do you have the same issue regardless of which you're trying to watch?
Also, if you press the MENU button on the TV remote (not the cable box remote), does the MENU come up bright as normal, or also dark (when the screen goes dark)?
This indicates a problem with the mainboard in the TV. If the MENU also got dark, then the backlight inverters would be the culprit. Seeing as the MENU is just as clear and bright as it was prior, then the panel, T-con (LCD controller), and the power supply are all likely fine. The mainboard is the board that routes all of the AV signals and processes them to be sent to the T con. There are no series of buttons you can press, or adjustments you can make to cure this problem. Average cost to replace the mainboard is around $350 parts and labor.
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