computer turns off suddenly

[etsh-eg]

Hi there

My computer I had built 3 years ago with the following specs:
Mobo gigabyte z77-d3h
Processor intel i5 3rd generation 3.0 @3.2 ghz
Ram 12 gb king stone (4+8)
Vga gigabyte nvidia 610 gt 2 gb ddr3
Hdd 1 tb western digital

The problem is:
My pc turns off suddenly and doesn't boot unless i unplug power cable, i checked temperature and it looks fine for me 55 degrees, so it isn't overheat issue, bios is up to date

 

[alexoiu]

Hi,
I'd test different power supply, firstly... if possible.

 

[etsh-eg]

Update:
I watched psu voltage crom bios and hwmonitor and found that the 12v rail increases from 12 to 13.9 constantly just before the pc turned off.
Could this be the reason?
If so can I control this? Or I should buy a new psu?
Thanks ^_^

 

[alexoiu]

You're welcome. What power supply using?

 

[etsh-eg]

The one which came in my case; cooler master 350 watts, it has been working well for 3 years.

 

[alexoiu]

Try borrowing one and testing.