If you solve this the CIA will probably hire you as their main IT expert. GET ready for an ESSAY, cause this will take a lot of reading, actually it won't.

Basically I had a budget so I bought all my parts except RAM and PSU used.

My specs are :

• GPU - GTX 670 Direct CU II
• CPU - I5-2500K no OC
• POWER SUPPLU - Seasonic S12 II 620W
• MOTHERBOARD - ASUS P8Z77 V LE PLUS
• RAM - HYPERX 4X2GB DDR3 RAM


So I built the PC, started having issues later, lots'a people blamed lots'a different parts, ended up flashing bios, problem almost solved, no more BSOD's, almost no more freezes. ( Occasional freeze when the PC boots and I don't use it for 2-5 minutes )
Now I got an HYPER 212 cooler, while installing it I placed the motherboard on my metal case, it slid around, touched metal, but in the end no capacitors were harmed ( I hope )
Every time I play any game, in 10-20 minutes the computer will hard freeze and won't react to my restart button, so it will be punished with me holding the power button down until the computer suffocates. When I want to turn it back on, it refuses to boot, just turns on the fans and stays there with no video or beeping, 10-20 seconds later it turns off, clearly cause it doesn't like me suffocating it. I wait around 1-2 minutes and it turns back on properly, with no more anger towards me. It boots, and if I don't play games it doesn't crash.
It also does a good job with no overheating out of rage on me. GPU and CPU temperature don't exceed 65C in both games and stress tests.
One thing I noted is that when it refused to boot, I took off the side panel, pressed the MemOK! button which loads fail-safe defaults if memory errors are doing something bad, in which case the computer forgot how to boot. I pressed it and it booted, never tried it again cause not playing any more games.

Stuff I tried :

• BurnInTest
• Prime95
• Bobby pin PSU thing
• Removing all parts and putting them back together ( not one by one ) twice.
• Re-applying thermal paste and re-seating the heat-sink thrice.
• Updating bios

Stuff I didn't yet try :

• Memtest86
• ( Whatever is left to try )


Also didn't try overclocking, which is the exact reason I bought the cooler for.

I don't know what else to tell you, unless you ask for information.

Forgot to add that it used to freeze on booting, and on windows screen or desktop the first couple of times I booted, from then it's just games.

You have not said anything about the Hard Drive.
Hard drives cause more trouble than anything else. Because that can be temperamental
Go borrow a good, modest hard drive and install your system on it. Don't use the other drive. Most of the time that solves boot up issues.
Quote

I placed the motherboard on my metal case, it slid around, touched metal, but in the end no capacitors were harmed ( I hope )

You have not said anything about the Hard Drive.
Hard drives cause more trouble than anything else. Because that can be temperamental
Go borrow a good, modest hard drive and install your system on it. Don't use the other drive. Most of the time that solves boot up issues.

Don't have anyone to borrow it from, I can do a SMART test though

very long and extensive real world experience has shown hard drives fail in ways you can not imagine. The SMART is not even doing 50%. The false positives and false negatives are about evenly divided. SMART has proven to be 50% worthless. You can do the same by flipping coin.
The test of a hard drive  is replace it with a new one.
Buy a new hard drive. The alternative is a new motherboard.

very long and extensive real world experience has shown hard drives fail in ways you can not imagine. The SMART is not even doing 50%. The false positives and false negatives are about evenly divided. SMART has proven to be 50% worthless. You can do the same by flipping coin.
The test of a hard drive  is replace it with a new one.
Buy a new hard drive. The alternative is a new motherboard.

...the operating system hard drive?

Yes. When the system  HDD does its dirty tricks the OS goes wild and unpredictable.
If the OS and the boot manager on are separate drives, it is hard to know what is going on when the system does a crash.

Take a spare HDD and do the full install from a DVD and install the chip set drivers.

An yes, a second D\HDD failing might bring down the system. It happens. Ofcourse it is rare. But then that's like declaring  accidents are rare.

Hi

I would suspect on a used motherboard that was that new that it has either had static damage which is impossible to find.

Or that when the board has been handled the chip set heat sink has come off and not reattached correctly. That heat sink should get warm if there is any conduction from the chip set.

Also if you shorted the motherboard to the case  there maybe incorrect data in the cmos . Have you reset the cmos  to factory defaults