PC randomly reboot when playing graphic intensive game

[whitefangvanish]

Hi everyone, my system has been reboot randomly while playing intensive game (Witcher 3). Sometimes it reboot after 15 mins, another time 2-3 hours. I've been noticed that after the suddenly shut down, my system showed a weird screen of inside my BIOS (only the tittle of BIOS are shown, the rest is only black) for like 2-3 sec then reset normally. When the problem first happened, I thought it was because overheat issue, and my CPU fan was quite old and noisy. So I replaced it by an ID Cooling FrostFlow 120L watercooling Kit. But the problem still persist. Now I don't know what the hell is wrong with my pc. Please help. Here's my spec:
OS: Windows 8.1 64bit
Motherboard: gigabyte 990fxa-ud3
CPU: Amd FX-8350
VGA: Msi R9 390
RAM: Mushkin Enhanced Redline Extreme 8GB x 2
PSU: Andyson N700 700W
I have 3x120mm fans and 1x140mm fan along with the watercooling kit.
The shutdown only happened when gaming. Everything's fine when I run photoshop, web-browser, watching movie. I did some search on internet, most of cases like mine are PSU problem. But my PSU is barely 6 months old. So I don't know, could it be drivers or BIOS problem ?

 

[CountMike]

Did you check memory and disk(s) ?

 

[whitefangvanish]

You mean my RAM memory and disk space ? There's lot of free space available in the drive I installed games, OS drive is the same. I also don't see anything wrong with my RAM. Their memory and bus are higher than the game's system recommendation.

 

[CountMike]

I meant to check if they are working correctly.