I have an Early 2013 15" Retina MBP which I bought refurbished from Apple about 2 years ago. Within the last couple of months the computer has been freezing up about once a fortnight at seemingly completely random times. There's no beachball or loss of display or anything, the computer just completely freezes up and any sound that was being played at the time continues to repeat on a one second loop. If I close the lid the apple logo stays lit and the computer doesn't go to sleep. I have to hold down the power button to turn the computer off and then I'm able to reboot it as normal. Once logged back in, a little pop up comes up and says that the computer had to be restarted because of a problem and gives me the option to reopen all applications - however, it doesn't provide me with any information on what the problem was. Computer specs are as listed below: 2.7 GHz Intel Core i7 16 GB 1600 MHz DDR3 Intel HD Graphics 4000 1536 MB 512 GB SSD (with about 140 GB free) I'm currently running El Capitan 10.11.6 - I'm can't remember exactly when the problem started occurring but I'm almost certain it never happened before I upgraded to El Capitan. But i'm also not sure that it started happening immediately as soon as El Capitan was installed... I tried to use Apple Hardware Test to check if it was a RAM / hardware issue however the test would keep freezing after 19 seconds so I was never able to actually test anything. I've run memtest twice using Terminal in the fully booted OS which hasn't found any issues. I've tried running it in Single-User mode however when doing so it comes up “Build User(): error building a user of type 0x20010008” and then continues on in Read Only mode which doesn’t allow me to run memtest.