PC starts to freeze 4-5 mins into a game

[Mikaseis]

Current build:
OS: Win7 Professional 64bit
Motherboard: ASUS M5A99FX PRO R2.0
CPU: AMD FX-9370
GPU: GIGABYTE GTX 760 WindForce R2.0
RAM: 2x4GB HyperX Kingston Blu + 2x2GB Corsair Vengeance(From old pc)
HDD: SAMSUNG HD103UJ (From old pc)
PSU: Hantol 800w (Surprisingly it ain't on fire yet)
Cooling: Seidon 120m
Case: CM 912 HAF+
This entire combo btw is prebuilt, a year has passed with no problems at all, I did not build it myself, even though I should have. Build has 2 monitors, main one 27inch and left side 24inch.

So my pc keeps freezing after a small cleanup I did today. It follows:
Re-seating gpu, moving a few cables around to fit em nice and snugly, turned over the psu so that the fan faces downwards, re-seated 24 and 8 pin cables from motherboard. After this, I closed up the case and connected everything as it was before. Upon starting the pc, everything is fine. I turn on Steam, Winamp and Chrome as per usual. After Steam launches, I turn off Chrome and start up any game (Lets take TERA for ex.) and let it start up. (NOTE: Even though dual monitor, main one plays the game, secondary is the extended desktop.) After I enter the game, 4-5 minutes into the game, the entire PC freezes and it doesn't respond to anything. I need to restart it every time this happens. Weird part, this happens only while a game is launched, otherwise the PC runs fine.

What I've tried:
-CHKDSK(HDD is from an old pc,had bad sectors before, but Hiren's solved it, for now)
-MEMTEST(Ram works fine, voltage is equal)
-Driver updates on GPU
-Windows Update(It had some update to finish, I let it restart but problem persists)
-Reopened case and refitted everything in it's place

Help?

 

[parik]

Do you use game booster?

 

[Mikaseis]

Nothing, I use no external apps to modify any values whatsoever, the only app that runs with the mobo is the AI Suite II and I've used it once to modify fan speed.

 

[parik]

Is it the same case for all the games?if yes,try updating your graphic card drivers or reinstalling them.

 

[Mikaseis]

Yeah it applies for every game I've tried, specifically tf2, tera, csgo, etc. I've tried rolling back/re-install/update.

 

[Mikaseis]

Ok get this. PSU was flipped upside down post cleaning cause I thought the fan facing the floor should be better since it has open space to drain air from, instead of looking upwards at the GPU. PSU wasn't made for being flipped. Flipped it back into same position. PC works fine. Conclusion: PSU is older than an MSX.

 

[parik]

good luck