Chosen Solution
I have tried the following, with limited success: The first few times it crashed, I ran disk Utility, it found no problems with the hard drive. Tried booting from the emergency boot disk, had similar problems. Reinstalled OS (High Sierra), no change. Moved back to Sierra, lost emergency boot disk, but stopped crashing for a week. After the week, crashes immediately on 75% of start ups, but ran ok once it ran. Now it crashes on 95-99% of start ups, and even if it starts ok, it will crash for no reason after running normally. There is no pattern (using the same programs, sometimes the crash-the screen turning scrambled then solid black or white, sometimes I could use it with no problems), and I haven’t gotten it to start up in the last 48 hours. Flashing PRAM doesn’t change anything, holding option to do an Internet restore may finish 50% of the time, but doesn’t change anything. Starting it from a different disk doesn’t change anything. It crashes two ways: 1) it hangs during startup and simply starts over, 2) it starts up and never finishes, sometimes with the apple and progress bar disappearing.
Sounds like your dedicated GPU has gone. Try booting up in diagnostics and see if it passes. Mac startup key combinations Let us know what you find.
