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Most often, this symptom is a result of a failing POWER SUPPLY in your TV, although other causes are certainly possible.
One quick way to check this is to remove the rear access panel on the TV and visually inspect the power supply circuit board for any scorched / bulging / leaking electrolytic capacitors. If you see any evidence of this, then replace the power supply circuit board with a new one, since this is the most often the source of the trouble.
Here are a few links to images which show what faulty capacitors often look like:http://www.facilitateit.com.au/Capacitors.htmhttp://www.pctips.org.uk/html/problems.html#Plague
You can usually find replacement power supply circuit boards online by searching for your TV's model number.
Here are some links to replacement POWER SUPPLY BOARDS for your model of TV currently available on Ebay for prices ranging between $76 and $126 US plus shipping:http://www.ebay.com/itm/LG-42LC2D-42LC2D-UD-board-6709900017A-/120746990663?pt=LH_DefaultDomain_0&hash=item1c1d14dc47http://www.ebay.com/itm/LG-EAY32731102-6709900017C-SMPS-Power-Supply-Board-/170641303008?pt=LH_DefaultDomain_210&hash=item27bb03a5e0
Just use your mouse to click on the link, which is the underlined and highlighted text just above. This will open a new web browser page automatically for you and allow you to view the information and items at the website indicated.
The repair / replacement procedure is not extremely difficult if you have some basic electrical safety knowledge and some basic electronics / mechanical repair skills.
You will need to unplug the power cord from the unit, lay the TV screen side down on something soft and flat like a bed if necessary, and then remove the rear access panel or the entire rear cover as required in order to access the circuit boards inside.
When you remove the rear cover and access the circuit boards inside, just check the part number that is printed right on the circuit board currently fitted in your TV and confirm that it EXACTLY matches the ones provided in the online listings. There are often different parts used even within the same model number, so be sure the part numbers fully match. Also make sure that any part you order is labelled "TESTED WORKING" in the condition description. Any part with a condition description of "AS IS" is probably faulty itself as well.
Then just unclip and remove the various electrical wire connectors leading to the power supply circuit board. Then remove the screws securing the circuit board to the TV frame and then remove the power supply circuit board. Be sure to avoid touching any large capacitors on the circuit board or the contacts on the underside of the board which can still retain a nasty charge even after the unit is unplugged.
Then just screw in the new circuit board and connect all the wire bundles to their appropriate connectors on the new circuit board and then replace the rear cover. Then plug in the power cord and test.
If you are uncomfortable at all carrying out this level of repair yourself, then you will likely need to contact a local shop to arrange for service. Depending on the shop and exactly what they find during their troubleshooting, the bill is likely to be at least $200 if you do need a technician to carry out those repairs for you.
Obviously if your TV set is new enough to still be under warranty, then definitely do not open it up or attempt to service it yourself, which will certainly void the warranty. In that case just contact your dealer at the place of purchase and arrange for a refund / exchange / or no cost warranty repair.
I hope that was helpful to you, good luck.