[Solved] Power Issue

So I've gotten myself a new system, everything was running fine except very seldom occurrence of my computer just shutting down by itself. (I'm still stuck with this issue).

I've went ahead an overclocked my processor (nothing major, voltage and everything on stock), ran test with prime and it was fine even after 15 hours of all 3 options.

It ran fine for a few days, until I wokeup one day to realize that I was unable to powerup computer. I've opened it up to observe the problem and I've realize that the first thing that happens is that the power comes on for about 0.5 seconds, fans spinning and stuff. But the power goes off and I am unable to power it up anymore. I've tried replugging the processor and mobo power, the same thing happens. Another point is that sometimes if I hold on to the power button for a slightly longer period of time, it boots up and works fine. (Ran for about 8 hours before I turned it off and went to bed.)

Things I've done are
Use the z68 onboard gfx instead of my 7970, same issue.
Use my old 8800gt, same issue.
Used a old psu, same issue (greater success rate with the random booting)
Using the old psu with the z68 gfx, same issue
Using the old psu with the 8800gt, same issue
Reseated the cooler fan (reapplied my thermal)

Computer Specs (Should have this at the top but I forgot and I'm on my iPhone so sorry)
Processor: i5 2500k @ 4.3ghz
Motherboard: MSI Z68A-GD65 (G3)
CPU cooler: noctua nd-d14
Graphics card: sapphire hd7970
Ram: G.Skill 1333mhz 2x4gb (black pcb)
PSU: Seasonic X-Series 760w
Storage: Crucial m4 128gb sata3
1Tb Samsung sata2
1tb caviar black sata 3
Chassis: NZXT Phantom USB3 edition

MOST IMPORTANT, a slight bit of coke was spilt into the fan at the top, I'm not sure if it has dropped on anything. (The system was running fine after the small spill, but problem started occurring the next day).

I'm fine with replacing whatever is spoilt, (the person who spilt the coke on my system is paying). But I need help with fixing it or finding out what is spoilt so I can repair/replace. Thanks!

run memtet86+ overnight

Do note that I've repeatedly try to turn it on and im not meeting with any success. And I doubt it is a heat problem when prime ran fine for so long. Temps were fine too, they were siting at 39-41 Degree C at windows.

Now after replugging the motherboard and processor cables, it's turning on for 3 seconds then off, and on and off. All by itself. I'm not even sure the drink spilt on any components, the most possible location would be the nh-d14 but it was clean when I took it out.

Oh and yes, that means I'm unable to boot into windows so any test is not possible

Ohey, that's a real smart idea, I'll set off to do that now

Motherboard is bad.

Everything was out, only the mobo and psu.

Alright, I'm outside right now but I'll try that out. And by putting the CPU back in, do I put my cooler as well or just the i5 chip?

Pretty sure it's clear of screws. I will double check that too.

Thanks for the help so far! Glad I'm not stuck fixing it alone!

Will do~ As soon as I get back home when my night class ends!

I've already prepared the necessary stuff for RMA-ing my mobo, and if needed my PSU.

Ah crap, that means I have to reapply thermal paste, wonder if I had any more lint free cloth around

Ouhhhhh, alrighty thens, I'll do this within the hour and post back my results.

Ok, got my motherboard replaced, EVERYTHING is working fine!
I am now stress testing my i5 at 4.5ghz.

Thanks for all the help, everyone who did.

And madmax, really really big thank you to you for all that you've helped me with! CHEERS!

Unable to select best answer on both IE and firefox

Great! I'm glad you got it worked out well, happy computeing!