You haven't posted your system specs as requested. Regardless of it's age, PSU may be poor quality. Hence the request for system specs.
CPU: I5 4690k
CPU cooler: Can't check it atm. I think it's the one you get it with the CPU.
Motherboard: AsRock Z97 Anniversary
Ram: 16G dual (8 each)
SSD/HDD: Kingston 250GB and 500GB
GPU: GTX 1070 8GB
PSU: Seasonic S12III-650, 80+ Bronze, 650W
Chassis: Zalman. Which exactly, forgot
OS: Win10 Pro
Monitor: LG Led monitor
Maybe this info is enough? Seems the shutdown happens mostly when i play Guild Wars 2, for example, but even when playing Last of Us. Initially i thought it's cause of multiplayer games, but seems not. I played even more demanding MP games, yet i did not encountered such problem like in GW2. Still, playing LoU, very demanding game...and bad ported, also makes my PC shuts down if i play like 2 hours. Funny though that BF2042, demanding too, does not cause me such problem. It's kinda ironic and weird atm, making hard to determine which component is the troublemaker. Also, did monitoring the temp of motherboard 36-37 out of 38 when literally just browsing internet and not playing at all. Is this all right? As with the PSU, cooling fans if i read it correctly, 1099 out of 1180, average 1063.