Picture and sound remain on roe only a few minutes,then go blank while power is still on.

Please help me with this lg tv 42LB1DR because "life is NOT good" and lg electronics inc does not rate too high in my experience thus far with them.The tv will turn on and may have a pictuer and sound for a few minutes,but will go off although the green power light shows its still on. Tv picture and sound go off while green light remains on. Push button for tv to go off and there is a long pause before it will actually go off. I had main board replaced 3-4 months ago because of the same problem. Is there something else that could cause main board to go out?