Pc crashing mid-game

[Nitroscout]

Hello!
Here are my specs:
Operating System
Windows 10 Pro 64-bit
CPU
Intel Core i5 6600K @ 3.50GHz 47 °C
Skylake 14nm Technology
RAM
16,0GB Dual-Channel Unknown @ 1067MHz (15-15-15-36)
Motherboard
MSI Z170M MORTAR (MS-7972) (U3E1) 44 °C
Graphics
SyncMaster (1920x1200@59Hz)
4095MB NVIDIA GeForce GTX 1070 (ASUStek Computer Inc) 46 °C
Storage
238GB ADATA SP900 (SSD) 34 °C
931GB Western Digital WDC WD10EZEX-08M2NA0 (SATA) 29 °C
Optical Drives
No optical disk drives detected
Audio
Realtek High Definition Audio
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So the problem is that when i play 3-5 hours (any game) my pc just freezes, temps are always below 70, and to restart pc i need to hold power button down for 5 seconds

 

[Hardware Brad]

You can test the CPU for 3-5 hours running Prime95 and see if it crashes. If it does it may be CPU, RAM, or MOBO related issue. If Prime95 does not lock up after several hours, run Furmark for 3-5 hours. This will stress the GPU only. If it locks up during that, it may be GPU related.

I'd also make sure video drivers are up to date.