PC shuts down with graphically-intense game

I'll run the tests and report back. Thank you both!

I ran Prime95. My CPU would not go higher than 50 Celcius at 100%. It's fine.

I ran Furmark @1920x1200, no AA, everything left at default. I've let it run up to ~15 000 frames after seeing that the temp won't go higher than 75 Celcius. I was relieved...... until I wanted to exit Furmark by pressing Escape, and then the computer shut down.

Update: as time went on, the problem stayed, sometimes it vanished for a week or two. Things started to get out of hand back in May; when I opened up Mozilla, the PC would restart. No warning message, no information left in task scheduler. Nothing.

On the day a restart happened, the frequency of further restarts were extremely high. The first restart happened completely random. So we called a tech-guy out, and ironically the computer wasn't doing restarts then. But there is an absolute way to make it restart by running Furmark and opening up a Youtube video. It's guaranteed.

I cannot afford to be without my PC because I need it for work. Two weeks ago it became unusuable as it started restarting every-, every other minute. I took it to the local service, and got it back today. The guys told me that it was the GPU. They ran some kind of enthusiast-level test on the GPU (something that usually takes 4-6 hours at least), and it only lasted 30 minutes. Temperature changed with every minute until it topped out at 81 Celcius, and the computer restarted. They told me that the GPU, and the PCI slot (where the GPU resides) get extremely hot for some reason. They took out the GPU, and the computer was working flawlessly. Told me to send in the GTX 970 for an RMA, meanwhile the onboard Intel graphics card will do.

So I brought my PC home, took out the Geforce, enabled onboard graphics and it was working fine.............for an hour. Then it restarted. Then worked for another 15 minutes, then restarted and restarted after another.....

I tried so hard to isolate the problem. I'd have to send the whole computer back to the capital city (where I purchased it from), and they would sit on it for at least a month. Even two weeks out would cost me my job. Of course, I should have thought of a backup PC first, but it's too late for that. I haven't worked in 2 weeks already.

Lastly, I talked to our IT guy and he recommended that I take the side panel off, and put a huge fan next to it, directly aimed at the interior of the PC. So I did that, and it's been an hour. I've opened up Firefox and had 10x more tabs open than I usually do, and had several youtube videos running, Skype, Photoshop etc. it's been working flawlessly. One odd thing I noticed with video play (such as Youtube), that the video is now silky smooth. Like it always had some kind of micro stutter, but hard to notice if you're not used to smooth playback. Such stuttering (but in a more aggressive way) was also present the moment I bought the computer and played CS:Go. I don't play anymore so I can't tell if that changed with the "extra cooling".

The computer is not overclocked. I actually had to disable Turbo right at the start, because it raised my temps to 100 Celcius. When I mentioned this to the service guy, he was like "well, yeah... what did you expect from a Hyper 212? You would at least need a Noctua for an i7"...

Another new information I could provide is that my RAM is not on the supported list. It supports Corsair CML8 and CMZ, but mine is "CORSAIR 16GB Vengeance LP DDR3 1600MHz CL9 CML16GX3M2A1600C9". BIOS shows 1333 Mhz, but the Corsair box says 1600 Mhz. Our IT guy said this shouldn't be the source of the problem.

So now I'm sitting with a big a** fan next to me, set on high, making me freeze to death, but keeping the computer stable. Even if the fan wasn't on, my CPU wouldn't go above 60 Celcius with stress testing. I don't know what causes the problem anymore. This is one random, mysterious nightmare. If anyone have heard of such thing before, and could help isolate the problem, then I would be forever grateful!

edit: I mean I get the part that my PC may not be cooled optimally, but having a fan the size of a tire running all day is not an option for me. Forgot to add that the guys in the service tried putting a side fan on the case for cooling, and it didn't even touch it. Same heat. I really don't want to do water cooling for a stock PC with such a highly regarded case such as the R5. The problem has to be isolated.

Just wanted to update everyone that this long-winded goose chase finally came to an end. The problem was with the PSU. So now I'm on a hunt for a new "reliable" PSU. The PCI slot on the motherboard (of the GPU) was discolored because it overheated so badly and so on (things that I'll have to live with and cannot get replaced with the shop).
One thing is sure. I've had PSU failures in the past, but non of them produced such sneaky symptoms as the current one.... leading people to believe other things were at fault starting from GPU to RAM to CPU to lack of air flow etc. I'll never buy Seasonic ever again nor shop from the same store.

Just a little update: my PC has been sitting in the service for OVER a month now. The guys who put my PC together admittedly applied crappy thermal paste on to the CPU socket from the very beginning, so they reapplied a higher quality Coolermaster one as I kept demanding the Turbo mystery to be checked, and according to them, the CPU now functions as it was meant to be (TURBO mode works without reaching 100 Celcius).

They diagnosed the issue (faulty PSU) the first week, and sent it back to the manufacturer the same week. Like I said, my PC has been sitting there for over a month now. Decided to contact Seasonic earlier this week, and demanded answers regarding the whereabouts-, and status of my PSU. No answer. Nada. Before you say anything, yes, I was polite and thorough.

Elbert, I'll grab the EVGA next week because my career depends on getting back a fully functional and speedy computer. No more Seasonic nor any associates with the bunch (such as Corsair).

As a side note: I didn't get why people went with building their PC-s themselves......because e.g. applying thermal paste the wrong way could make or break things, and it's best to leave it to a "professional". It took me quite a while to understand. My professionals were a joke. Wish I knew more about this back then. As soon as I get my computer back, the first thing I'll do will be to develop my PC building skills to an expert level, and never buy a complete PC again. I'm pissed off.... can you tell? This has been a complete nightmare. So upset with myself for chickening out on building everything my own.