[Problem] LG TV 42" model 42LS5700-TB: Black screen with sound

I am looking for advice about what is wrong with my TV: it turns on and you see the LG logo, everything works fine until after 10-15 mins of running, the screen goes black but the sound still works fine. One can see a faint picture on TV when light is shining on it. So sometimes I hold the power button for 10s, turn it on, and it works all fine again, but goes back to black screen after 10 mins. Sometimes it turns on, shows the LG logo then goes straight to the black screen. Anyone know what is going on?