this is a notebook, right ?
when the screen is dark , look really carefully maybe with a bright external light and see if you can see the desktop or what ever was on the screen when it went dark. If you can see the desktop or whatever it is the inverter or the little fluoresent tube. The inverter is a circuit board outside of the lcd module, in the lcd case. The tube is internal. If the screen goes to bright w/o the desktop or whatever, it could be the LCD, Cable, or onboard video. If you have a vga out on the notebook. Hook up a desktop monitor and run it for awhile and see if you get the same results. If not that eliminates the onboard vga. You may have to go into the bios to set the notebook to use the vga port.