It could be the GPU,CPU or power supply. Its a fail safe so your motherboard doesn't burn up, thats why it shuts off. It could also be a broken or loose part that can be replaced. I would buy another one and keep this for parts. Fixing the CPU of GPU is a $8 thermal compound fix. Thermal compound is like car radiator fluid (keeps it cool). I have no right answers because I don't have it to take it apart to see but this is a overall idea of the problem.