There are a couple of things that could cause this, unfortunately there is no reset for this televisions and it will have an internal failure at this point. I will try to explain the possibilities the best I can. What you are deciding on is a very hard thing, I am not sure but I think I would return it if I were you, the price of some of the parts in the set are outrageous and if we determine one of them has failed it would be a bad thing. With that said there are some minor problems in the power supply that COULD cause this but not guaranteed.
First you could have a failure in the power supply, we have seen some capacitors fail in your model and cause funky problems like this. If this is the case you could be looking at somewhere around $175-$200 to fix the set. IF the power supply had to be replaced you could be looking at $350
Next could be the main board, this would not be good news because you could be looking at $400-$500 for the repair (I know this is up there in the price range and most likely getting over the cost of a new set)
Lastly you could have a problem with the panel itself, this would mean a new set for sure because the part cost is more than a new set. The panel in your tv is about $800 just for the part.
Any of these could cause your problem, but it will require troubleshooting to determine the exact cause of your problem. If you have experience, and test equipment and would like to attempt some troubleshooting I will be happy to assist you, however if you do not have equipment or experience then you would need a repairman at this time to get an exact diagnosis. I am sorry that I don't have a quick fix or easy answer for you on this one, it is definitely an internal failure.
If you would like any additional information or have more questions please don't hesitate to ask!
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