Precision T7500 loses power

Hi all,

My T7500 developed a problem which it'll just lose all power all of a sudden with no signs. 

On the first start up after 3 month it worked fine for 30 minutes, then it died. Then the power will come back on after 10 seconds and is able to start agian. (Was just watching a YT video on Chrome when this happened)

The second try Windows is fully loaded but dies after about a few seconds.

The third try it'll only have power till the Windows loading screen then dies.

Subsequent tries will only have power till Bios screen then dies. And it'll get worst as power will only last for about 5-15 seconds upon starting.

The second day the PC actually lasted for a few hours before it dies, subsequent starts it'll just die during Bios booting up.

The PC is in a 10-20c room.

The T7500 was unplugged 3 months.

The CMOS battery is still good, the date and time is still in sync.

Connected & isolated BIST test on the PSU shows a solid green light. (However sometimes I can kept BIST pressed for hours and it'll run the test with the green light before losing power, sometimes it'll only last for a few minutes before losing power.)

Specs:
Intel Xeon X5667 (2 processors)

Radeon RX 580 ARMOR 8G OC

Dell's T7500 1100w stock PSU

20GB DDR3 ram

1 SSD, 1 HDD

It seems like it could be the PSU, but I like to have others opinion as what else could be at fault here, thanks.