Bought a used 2012 rMBP (GeForce 650M 1 GB / Intel 4000) earlier this year, and it has been working just fine, then all of a sudden starting crashing the instant I connect an external display; or after a seemingly random amount of time (from seconds to an hour) while running any app that forces the nVidia GPU to turn on. When this happens, the screen stays dark until I preset PRAM, sometimes it takes multiple tries. (Cmd-Opt-P-R.) I figured it was the now-expired-for-my-model Repair Extension for Video Issues, so I figured I'd give it a go and bring it in to an Apple Store for that issue. Brought it in, demonstrated it with a GPU test app (FurMark, crashed instantly.) Tech booted from his diagnostic image, passed all the tests. Ran the diagnostic again, passed. Booted to their troubleshooting copy of macOS Sierra, plugged in an external display, worked just fine. Ran the same GPU stress test app, ran for many minutes just fine. He checked my logs (as I had) and it was definitely always GPU-related kernel faults. After running GPU stress tests repeatedly, with an external display plugged in, came to the conclusion that it was software, not hardware. Although the service history did indicate that it HAD been brought in for "video issues" in 2015 (when the repair extension was still in force,) and at THAT time, they determined it was just "poor cooling" and replaced one of the fans. That did seem a little suspicious to me. I felt like an idiot. So now I have to reload the OS. Brought it home, made a restore USB, and the installer fails before even getting to the GUI, same behavior as I saw before - screen shuts off, I have to reset the PRAM multiple times to get it to come back. Now I can't even get it to boot properly 90% of the time. Oddly, I can boot my iMac's OS if I put the iMac in Thunderbolt Disk Mode and connect it to the MBP. (When it's willing to boot at all.) Anyone have any ideas? Is it actually hardware, and I just happened to get (un)lucky when I brought it in?