If the MacBook restarts (kernel panics) or image goes out (but fans still spinning) it is likely an issue with the soldering on U8900, common problem on this model. If the image goes out but backlight is still there, or it is completely turning off, it is the GPU most likely.
1st scenario, reflowing the solder around U8900 with leaded solder will fix the issue; the joints crack due to the heat from the CPU and GPU on the other side of the board. 2nd scenario, contact Apple and if they won't help, I wouldn't recommend repair. Replacement logic boards have a high chance of developing the same issue, and there are no good GPUs for these anymore.