PC Restart, it really confuse me

[aldieringo]

So i have been looking for some threads all over the sites for weeks but i still can't find the exact same problem as my PC does.

my details:
core i7 4770
EnLight 500watt
Radeon 7850 OC 2GB
Gigabyte Mobo GA P85 D3
8GB of RAM 1 stick
they all are 5 years old, but i don't want them to just give up yet.

my problem:
my PC keep restarting at random time such as
- previewing high resolution photos with Widows Picture Viewer
- rendering video
- watching web-video
- using Windows System Rating Feature
- when i turned on the PC, the few seconds after it show the desktop i open few applications it goes unexpected restart, but if i wait until all the support software loaded it does not restart

i tried to replace the RAM, the VGA, the Hard Drive, checking Processor health, checking the motherboard even the PSU
they have no issue. they aren't overheated too.


my question
did i miss some important things?

 

[Vic 40]

How did you check that psu?

 

[aldieringo]

How did you check that psu?

plug it on another PC. my friend's PC to be exact.

 

[Vic 40]

Tested with his psu on your ystem?

 

[aldieringo]

actually i found my problem. already fixed it, i replaced a new battery stuff inside my motherboard.