Blacklight, temperature, lcd, screen,

Hi guys

I have a problem with my iMac i7 27" late 2009.

sometimes the screen goes black for no reason. I try to put the flash from my iPhone on the corner of the screen and I can see the icons. Every time that I turn on my iMac after few minutes the screen goes black again. I leave the iMac until stand by and I come back to it after 10 minutes, the screen work again but after few minutes black again. I was thinking there are some problems with temperature sensor or my back light led doesn't work properly. I will insert few pictures from temperature gauge if you would to have a look.

Those picture are about 10 minutes working.

After those one just for the LCD temperature it's grow up in about 10 minutes.

Please let me know and help me to fix this problem.