I have a 2011 17" MacBookPro - 2.2 GHz - 16GB Ram - Samsung 850 EVO SSD 500GB - El Capitan 10.11.3 Never had a problem until last night and while in Safari, when I clicked on something the screen went black? Had no choice but to hold down the power button to shut down. Restarted, no problem. In Safari about 2 hours later, screen went black again after watching several tutorial videos. After shutting down, I reset the PRAM upon restart and desktop appeared as usual. I then shutdown and unplugged the power supply overnight. Today I reattached the power cord, fired it up and after the Apple appeared and the Progress Bar... black screen instead of desktop! Rebooted using Command-S and did fsck -fy - all showed good. Rebooted, black screen? Tried several shutdown cycles like this and it finally came back up (am typing this message on it now). So what would cause this? The screen does not appear to be bad, just the desktop goes black when it loads sometimes after startup or in Safari (so far).