5 sec is all you get

[Sheepykins]

If this were a bad memory issue the post would alert with beeps usually, but its strange ....so you're saying the whole thing (system) powers up for 5 seconds and then turns itself off?

Unplug the hard-drive and try to turn it on, as long as your bios is set up right it should not just switch itself off after 5 seconds, this would indicate a power issue.
Along with the PSU, replace the mains cable you're using.

But honestly my first instinct is temperature as wertz suggested, it fits the profile! if not, mobo is a lemon

 

[Digerati]

But honestly my first instinct is temperature

I don't see it being heat in 5 seconds - not unless the heatsink was not properly mounted and/or the TIM (thermal interface material) was improperly applied.

But then as I note in my sig, it is always heat. So a failing component just cannot handle the expected current/heat. That said, a failing component may be drawing too much current, causing the PSU to fault.