I Googled, I searched Apple forums, but I can’t find anything relevant enough, so I thought to seek the wisdom and experience of this group.
MacBook Pro 15-inch early 2011 with 16 GB of RAM running El Capitan
I love that computer and it has been my main workhorse for years. During the last year, it had several instances when the screen would suddenly go white or black and computer would stop responding, while I was working on it. After forced shut off, I could reboot. On occasion, the computer would hang up during booting. However, until now, I could always start from my emergency USB drive, run checks and then restart normally. Nothing ever found except occasional corruption of drive directory, nicely fixed by DiskWarrior.
Past few days, the computer fails to boot. The boot progress bar gets to almost 2/3, and then
a. I see a stripe of funky lines about 1 cm high in the upper part of the screen, the rest of the screen turns white, and after some 10 seconds computer self reboots
b. the screen turn white, some times black, even more seldom blue, the computer sits for like 30 secs, then reboots (usually, sometimes it just sits like that for ever)
Holding D at boot, brings hardware diagnostics fine but even running the extended check a few times finds no problems.
Holding shift to boot in safe mode does the same as above but never self reboots.
Holding cmd-R does the same when the boot progress bar is about half way. No self reboot.
Holding cmd-V to get verbose boot, the last entry is appleLPC:notify…
Resetting PRAM reduced frequency of hang-ups, that is computer reboots self more often.
Booting from a USB stick (holding the option key to select the boot devices) does the same as booting from the main drive. The USB drive is a Yosemite-based drive with utility tools created by DiskToolPro.
The moment the computer gets weird during boot is the same when during normal boot the screen blinks (progress bar disappears for a second, then comes back).