Question PC suddenly turn off and wont turn on

[qqsquare]

Hi all

The PC been using for 2-3years and suddenly random turned off and wont turn back on. So after looking at web so i changed the mobo. It went well for 2-3 weeks without random turned off. Then now it started to turn off randomly. Then i bought new PSU it did not work. Take out the ram with one stick and took the GPU out, with on-board graphic. Still random turn off.
Memory one stick did memtest86 no error. Please help.

 

[qqsquare]

when old Zalman 500w PSU is used, random turn off and wont turn on again. after change new EVGA 600W GD it will start and run as normal but will still random turn off and will turn on after random turn off but after few hours use it wont turn on after random turn off. it will boot if i change back to old Zalman PSU.

 

[Zizo007]

What are the CPU temps in bios?

 

[qqsquare]

im currently testing CPU temps in bios, sitting at 40 C, testing staying bios for 1 hour or so now. no random turn off yet.

 

[qqsquare]

i dont think its CPU over heating, as when i change the mobo i reapply good grade thermal paste. should last for some time.