Computer shutdown....pulling out hair.

[lagagne]

K my computer was shutting down after a few minutes, sometimes when windows screen was booting and as far in as the desktop. I decided to reload windows entirely but its shutting down still after its on for a bit.
Ive tried everything i know to do. Heres what ive done:
Reseated cpu and heatsink...no overheating.
changed psu
changed dvd rom
pulled system down completely including MB
tried ram one at a time in different slots
checked onboard vpu for heat and its fine
all fans running and temps look good
unplugged the front panel usb and audio leads from case.
HELP plz lol

[Whyzman]

By shutting down, do you mean complete loss of power...not just a freeze up?

[123456]

Ahhhhhh! When you reseated your heatsink, did you remove and then add a new layer of thermal compound?

[lagagne]

No power. And yes always use artic silver compound.

[mjc]

Are all the power cables properly reconnected?

[66miranda]

might be for sure. .