Computer shuts down without warning

So I've been having the same problem as some other people in this forum I've noticed. I just bought a whole bunch of new components and used a few from my old PC, pretty much just the PSU being the only one of significance. Everything seems to be working fine normally but every now and again I'll be in a game and the whole PC just shuts off without warning. I've tried using WhoCrashed but there are no logs being made of these crashes. I only have errors on windows reporting saying windows was not shut down properly. But it is a catch all error and I don't know if it's stemming from heat issues, hardware issues, or power issues. Or if it could be something besides those. Here is what I can tell you: It only happens when I'm in a game, although that is 99% of the time since I almost exclusively game. But never when I'm on my browser or watching movies. Also, I have run memtests and my RAM appears to be all right. I thought it might be a power problem because although I have an 850 watt PSU, it is a corsair (Corsair HX850W) and I heard some people saying they are not very good? I would get a new PSU if I was sure it was that, but I'd like to be sure. I have many monitoring programs like OCCT, WhoCrashed, Speccy, RealTemp, and CPUID hardware monitor, OpenHarddwareMonitor, and Prime 95. Let me know if you want screenshots or log from any if that helps. Here is my speccy profile: http://speccy.piriform.com/results/bpZfMwd6a1BRgmmRlESRtfp

Thanks in advance for any help.

(If you are using nVidia) Roll your Video Card driver back to the 361.6 something and see if that helps. I just got a EVGA 950 ssc and installed it 4 days ago, but before that I was having crashes (un-identified) with my dual GTX 480's and after a driver rollback it stopped. Hope you get it solved. PeAcE

PS remember to use something like Ccleaner to cleanup and do a clean driver install.

Edited by TheCyberGhosT, 21 February 2016 - 05:26 AM.

Do not use CClearner to do a clean driver install. It will not help with video card drivers. What you want is Wagnard's Display Driver Uninstaller. Available here.

If you have a computer that suddenly shutsdown, like no BSOD just sudden power loss, it is almost always the PSU. It can be the mobo, but it is not as likely.

Corsair is apparently not a very good brand, at least anymore. I have been having problems with my Corsair PSU recently, and my friend had to warranty his. It was about a month old.

How old is that PSU? if it came from the old PC build then at this point it may need to be replaced.  This issue sounds like a PSU problem or as Gorbulan pointed out it could be driver related.  Usually though a driver related problem will give a BSOD before just shutting your computer down.