Before you commit money to repair, uninstall the nVidia drivers and run in vanilla VGA mode (or boot in Safe Mode and run that way). Does the fault show? If it does not, it's just about 100% certain the problem is the video chip - not the LCD. These nVIdia chips can -- and have -- failed in such a way as to render the circuitry that feeds the internal display faulty, while an external screen will show a good image for a while before it too shows faulty.
If so, you need a replacement mainboard - and nVidia never really fixed these chips, so any replacement board is simply a ticking time bomb. If you want to repair the system - replace the board with one that uses Intel video. Another nVidia board will simply fail again. You will need a new heatsink assembly to change from the unreliable nVidia chip to an Intel board.