MacBook Pro - not booting - going around in loops? After a harrowing couple of days, I found this site and am hoping someone can help.

I have a 2011 MacBook Pro which has recently started having problems booting. During a game session (SIMMS), it suddenly shutdown and refused to reboot, and since then I've been unable to use it at all. The MBP has a new install of El Capitan on it but it's been working fine for the last couple of weeks since the install.

I tried Safemode, Recovery mode etc, but it wouldn't boot into any of these either. Eventually I set it to Target mode, and reloaded El Capitan from another "Companion" MacBook Pro. This process went without a hitch.

After loading it, I booted from the "companion" MacBook to make sure the OS was working OK, and it was!

I restarted the "Companion" MBP and ejected the faulty MBP, and tried to restart it normally. No go, still wouldn't boot even with the new OS.

I've tried a number of things to get it to boot, following this fresh install, but all that happening is that it'll get to the Apple Logo, try to boot, shutdown and then go around in circles.

Eventually it'll stop on the gray screen and the fans will startup and keep running.

I'm at the end of my knowledge, so any advice would be welcome!

Thanks
Have you had a luck with it yet? 

Sound like it may be similar to mine. Logic board for Mac Book Pro's sold between late 2011, whole 2012 and start 2013 are under Apple's Extended Warranty Program. Your symptoms are the same as what I had before mine completely died. Going in tomorrow to have my logic board replaced for free. Thanks Apple. 

Please see the link below. 

https://www.apple.com/support/macbookpro-videoissues/

Hope this helps.