PC shuts off when in game for awhile

[Flyingpigyz]

My rig before upgrade:
phenom x4 940 3.0 GHz
2x4Gb ddr2 samsung ram
geforce 9800 GT
motherboard A780GM-LE
PSU 500W(doubt its any good quality)
HDD maxtor 500gb
OS: Win 10

- decided to upgrade GPU to geforce 750 ti since i heard it uses less power then 9800gt(went on psu calc site and it confirmed ~40Watts less)
- also decided to get aftermarket cooler for CPU since i will have extra power
- so i installed it
- did a little stress test, every temp looks good(with aida64)
- then in game some shit started to happen, HDD every now and then would make grinding sound and like stop spinning then start up again(everything fine onscreen)
- now every like 15 min in game PC shuts off or just gets black screen until i restart

Im thinking its the PSU, but i have my doubts. I'm not sure how should I check is my PSU failing, or some other component, and if it was something other, PC wouldn't just shut off completely right?

And yea its doing that and I didn't even get to overclocking part

 

[Dark Lord of Tech]

First place to look is at the PSU , if it's a poor one replace.

Shutdowns like that can be power related.