I got my motherboard fixed by gigabyte yesterday, It worked fine for 4-6 hours and then switched off. Now when I press power, cpy fan spins for a sec and restarts.(and it keep happening)
 
The behavior is same with following scenario,
 
IN case
All connected,
cpu + mobo + psu + ram + gpu
cpu + mobo + psu + ram
cpu + mobo + psu 
 
Out of case,
cpu + mobo + psu + ram + gpu
cpu + mobo + psu + ram
cpu + mobo + psu 
 
Right now my motherboard is out of case; connected just to PSU and CPU. There is no beep coming so its not reaching the stage where it beeps for no RAM present.
My hunch is that it is my motherboard as it got repaired for same problem; but want to rule out its not my PSU. I was trying few LED lights in my case connected directly to PSU when this happen;
 
How can I ensure that the PSU is fine?
 
I tested it with the given tester and it starts fine, what else?
 
Thanks

If it starts fine with a different power supply, you hit it on the nose. Time for a new one.
 

>If the PSU starts with the jumper attached, then you're probably looking at a motherboard or CPU issue.
 
Yeah the PSU fan keeps spinning with jumper connected to mobo cable. I will see if I can find another PSU to test.