It's either the inverter or an internal issue with the GPU. Chances are that it is the GPU, as if it was the inverter you wouldn't be able to see anything at all (also rules out the back light). It's most likely an issue with the GPU or a circuit or 2 from the GPU, most likely caused by too much heat over time. If it is under warranty, send it in to HP and they'll fix it for free. If not, you can find a repair center and see what they have to say.