Question PC turns off and fans get loud

[UD97]

PC turns off suddenly while playing games, surfing on internet or just idling. Then fan, I guess it's CPU fan, starts working faster and gets really loud. Having this problem for two days. This happened 5 times since yesterday. Still thinking about what it can be but don't have any idea.
While playing games CPU temperature is between 73-76 celsius degrees but I don't think the problem is from that because PC turned off when I was watching a video and idling on desktop.
What's the problem? What is the missing part of this puzzle?
Thanks.

Specs:
i5-2500K with stock cooler (Changed thermal paste a week ago)
Asus P8Z68-V
Asus HD6950 DCU2
Seasonic 850W (80+ Bronze)
Using with Windows 8.1 which I installed a week ago.

 

[DonnyTechMaster]

The problem is probably due to heat.

Is the inside of your pc very dusty? You may not thinks it's a big deal, but trust me it definitely can be.

If it isn't dust then try going into the bios and make sure that throttling is enabled under your CPU settings. This will lower the clock of your cpu if it gets to a very high heat level.

 

[UD97]

No, not from heat. There is no dust inside case cause I cleaned it a week ago. Everything works perfect except when I open APB:R. PC turns off after 20-30 seconds when I open the game.
I tried all stress tests. Nothing is wrong with GPU neither CPU.
When PC turns off and fans go on, Capslock is not working. Why is that?

 

[mamasan2000]

My guess is the graphics card. Mine did it too, a HD7870. Black screen, fan spins up to 100% speed. Can only hard reboot computer.
What I did was to get MSI Afterburner, setup a custom Fan Curve. It took a bit of fiddling around to find a curve that doesn't induce lockups.

I have a staggered curve, looks like stairs (double-click in the fan curve-window to switch). That way the fan don't have to get signal every second how fast it is supposed to be spinning.
40% at 50 C, 50% at 60 C etc etc.

Hope this might help you.

 

[DonnyTechMaster]

Does it actually do a windows shutdown or does it just cut off completely?

 

[Shadz92]

My guess is the graphics card. Mine did it too, a HD7870. Black screen, fan spins up to 100% speed. Can only hard reboot computer.
What I did was to get MSI Afterburner, setup a custom Fan Curve. It took a bit of fiddling around to find a curve that doesn't induce lockups.

I have a staggered curve, looks like stairs (double-click in the fan curve-window to switch). That way the fan don't have to get signal every second how fast it is supposed to be spinning.
40% at 50 C, 50% at 60 C etc etc.

Hope this might help you.

I'm having the