PC Freezing and then USB ports die

[mTz]

Guys, I have been trying to troubleshoot a problem but so far I have gotten even more confused than anything else.

Here is how the situation is.

win 8.1
i5-3570k
intel dz77sl-50k board
corsair 4gb value 1600
HD 6570 2gb
cooler master 550 psu
HDD WD GREEN 1tb
CD/DVD Drive

The problem started around a week back, system would start(cold boot) up fine would run around an hour then it would freeze & a minute later mouse and keyboard would power off(no light). Ones this happens and I restart it, next freeze would happen around 15min,wont unfreeze on it own, after few minutes display would go dark and switch off (not proper shutdown)

I cant afford to change all components(already changed RAM), if we can narrow it down to one part that is most likely to be the culprit, I would be green.
This is a basic system for very basic use(Browsing, research, youtube, video streaming).

I suspected RAM as it mostly happened during youtube and flash video streaming(last half second of audio keeps stuttering) but it has also happened during simple browsing and Some times when processer load is high.

I don't have a restore point saved, few months back I used this system to root an android device, uninstalled all programs installed for it. couple of weeks ago changed the ups.
CPU and GPU temps are ok(CPU stays below and around 60)

This is what I have done so far:-

Re installed windows. (win couldn't repair itself)
Updated bios and drives(lan, gpu)
Took out hardware, cleaned and re seated them.
Noted freeze times and checked event logs(got kms power error in critical, cant remember exact name)
Ran memtest+ and memtest but both would always get stuck at different stages(shortest time I waited 20min longest 45min)
Processor diagnostics passed, HDD smart test passed but extended takes to long and system would freeze in between it.
Ran Ubuntu off usb to complete HDD test, it still froze.
Changed old Ram' timing and frequency(it shortened boot time and startup app load time).

If some one has faced this before or have any idea what is going on, please spare some time and help me out. Thank You.

P.S. There are no visible or audible hardware damage and no burning smell either.

 

[Mikel_4]

Either MB or failing power supply, Intel cram the cheapest VRM, chokes, and mosfet on their product.

 

[mTz]

Either MB or failing power supply, Intel cram the cheapest VRM, chokes, and mosfet on their product.

Mobo would me cheaper than psu but there aren't many options for 1155 any more. I really want to make sure HDD is not the problem. whole system was build in JAN 2013 except HDD and PSU which were from a previous build.