this is a common samsung problem due to the incorrect components being fitted to the board. If you switch off and back on it should work till the next time.
No easy fix other than have engineer look at it. Mine does exactly the same Contacted samsung they deny its a problem check out web sites search samsung tv problem you will soon get the bigger picture. It will cost £65. Samsung are really ignorant and deny that its their problem suggest you contact seller and quote sale of goods act (not fit for purpose) Usually find its about two years old just out of guarantee when off goes the picture see if you can get samsung engineer to fix but deduct cost if its down to their faulty board
tom
December 2010
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Try fixfault.com to fix televisions.