Question PC crashes when gaming (I have to hold down the power button to exit)

[Surzilla]

My PC crashes entirely after 15-60 minutes of playing (pretty) demanding games, like GTA V, Modern Warfare, Conan Exiles, ...
In games like Rocket League it doesn't crash or maybe after several hours sometimes.
My entire system becomes completely unresponsive: the audio stops, the screen freezes, buttons stop working (even ctrl+alt+del or alt+F4), the fans suddenly get very loud and I can only hold down the power button to quit. The crashes happen even quicker after I restart my PC and the game.

I did a CPU test, a RAM test, the FurMark GPU test and all of it was ok. My drivers are up to date. I really don't know what the culprit is.

Specs:

CPU Intel Core i7-7700K 4.2 GHz Quad-Core Processor
CPU cooler Scythe Mugen MAX 97.18 CFM CPU Cooler
Motherboard Gigabyte GA-Z170XP-SLI ATX LGA1151 Motherboard
RAM Corsair Vengeance LPX 16 GB (2 x 8 GB) DDR4-3000 CL15 Memory
SSD Samsung 850 EVO-Series 250 GB 2.5" Solid State Drive
HDD Seagate Barracuda 2 TB 3.5" 7200RPM Internal Hard Drive
GPU MSI GeForce GTX 1070 8 GB Video Card
OS Microsoft Windows 10 Home OEM 64-bit
PSU EVGA SuperNOVA G2 550 W 80+ Gold Certified Fully Modular ATX Power Supply

 

[Tioym]

PSU?

 

[Surzilla]

PSU?

Sorry, forgot that one.
PSU: EVGA SuperNOVA G2 550 W 80+ Gold Certified Fully Modular ATX Power Supply