Mobo suddenly turns off after 5mn, won't turn back on

there are a few things that it could be,
a psu issue, which you can check with this
http://www.youtube.com/watch?v=5FWXgQSokF4

memory failure(run if you can which becuase you cant startup its not likely you can but still)
http://www.memtest.org/

then, unfortunetely it could also just be that your mobo is fried... you will want to also just go ahead and try another gpu if you have one. But other than those things it is your motherboard... but because it wont even power up(fans and stuff) it sounds like your mobo is just roasted

Thanks for your reply.

I already did the PSU test and the PSU powers up. And, if I leave the power cord unplugged a few minutes, the mobo powers up fine. I suppose my PSU is good (and, its brand new).

I ran memtest86 as you suggested and it did report errors with the memory module.

Funny thing is, I saw the errors being reported and the mobo stayed on, and then tests went on and I wasn't getting any more errors, and after a few minutes the whole thing just went down as it usually does.

If my mobo is fried, then memtest86 could be reporting bogus memory errors, couldn't it ? Or, it could indeed be the memory... I can't think of any more test other than buy a new mobo or a new memory module....

Thanks again.

no i am almost positive its your memory, though the mobo may be it too... the reason i say this is because the mobo would be the reason for the shutdowns(power offs) and the memory errors also may be... if you have no memory your computer wont even post so maybe if it just has some fatal issues it wont start   well try the memory in different slots or busses one by one, to try and see if one of your sticks is good and will work in one of your slots/busses