Boots only half way

My screen suddenly went black. Now the computer won't boot up. It starts, with a very grainy screen, then moves on to the screen with the apple and the progress bar, still very grainy, then the screen goes white and sticks at that for a while, then all goes black and it tries to reboot from the beginning. And so on. It's the same with an external screen. The ext screen shows the same as the Mac screen, which is precious little, but the Mac just tries to reboot anyway. Seems to be a video controller problem. Does that require a new logic board? But why does the boot up fail and restart?

MacBook Pro A1286 2.5 ghz quad core, 8 GB RAM, Samsung EVO 500GB SSD, OSX 10.10.3