My Computer is restarting while I am playing games

[Karben]

Today I got a new computer
The specs are here
CPU: intel core i5 4690 3.3GHz
Memory: 8GBx2
VGA:GTX 970 4GB
HDD:1TB
PSU:Raidmax RX-1000ae
OS:Windows 8.1

And I downloaded a few games like Cities Skyline from steam.
When I run it just less than5 mins, my computer is restarting itself with no bsod no error ! Just like I pull out the plug!

Someone plz help me I searched google and I can't find solution.
Thank you

 

[Tmagic650]

Test the hard drive by going to Computer and right-click on the C drive. Select properties, tools check now. Put a check mark in both boxes, to set a disk check on the next start up. Please note that this check will take some time, just let it finish...

 

[Karben]

Test the hard drive by going to Computer and right-click on the C drive. Select properties, tools check now. Put a check mark in both boxes, to set a disk check on the next start up. Please note that this check will take some time, just let it finish...

Is this error checking?

 

[Tmagic650]

Error checking and possible repairing as long as you checked both boxes

 

[Karben]

Today I could play the game about 20mins, my pc still restarts itself though.
And I can play League of Legends with no problem.
What is the problem?
Thank you alot

 

[Tmagic650]

Did you build this computer yourself? It could be a incorrect bios setting dealing with CPU slowing or stepping settings