Back in December I moved my PC to a new case, and noticed loose heatsinks on the motherboard, but I thought nothing of it because I knew I was going to be upgrading soon after anyway. It had an AMD Phenom II X4 945 and some old Gigabyte motherboard then, and sometimes it would shut off randomly and reboot without any BSOD, etc while playing CSGO or Payday 2 about a month or so after the case change.
It was occasional at first, but become more and more common as time passed. I tried lowering the graphics settings in Payday 2 and that worked for a bit, but even with that it eventually became so bad that I couldn't even have the game open for a few minutes before my PC shut itself off again, even doing so on the main menu. I put it down to the old motherboard, so I upgraded sooner than I had planned to an ASRock H97M Pro4 and an Intel i5 4590, and everything was working great for a couple of weeks. Now recently it's begun to happen again, only more frequently than it did when it first started back with the old motherboard.
My temps are low, and most of my parts are reasonably new, so I have no idea what could be causing the issue.
Specs: Intel Core i5 4590 @ 3.3GHz (a few weeks old) Kingston HyperX Fury 2x4GB (8GB) @ 1866MHz (3 months old, same time as case upgrade) ASUS GTX 660 (a year old) ASRock H97M Pro4 (a few weeks old) Samsung 840 series 120GB SSD (18 months old) Seagate Barracuda SV35 1TB (18 months old) Corsair VS450 (8 months old) Cooler Master Hyper 212X (3 months old)
If you are overclocking your GPU at all. I have a feeling you need to upgrade your PSU. Even if you are not overclocking your GPU I still advise upgrading your PSU. This PSU would probably suit your needs and your budget. Im not trying to tell you "hey you should buy this!!" im just giving you my opinion of what I would do if I had your setup. Also I don't think the PSU you currently have is not efficient enough to do the things you want to do.
Truth... you're frying your computer like an egg trying to draw amperage, that psu is a blood from a stone scenario that would be fine without dedicated graphics. I never recommend anything under 600 watts for gaming just for the amperage on the +12v rail.