Desktop freezes and randomly restarts with graphic intensive activity

Hey guys,
I've had a look around the forums and haven't seen anything like my issue so I hope it's okay to post this.

I build a custom computer about a year ago, and everything has 3 years warranty.

About a month ago my computer started blue screening when I tried to play any games, and if I accidentally knocked the desk it would freeze. It then completely stopped turning on, even though the actual computer was running... nothing was showing on the monitor.

However when the graphics card was taken out, it started up fine. So my mate put it down to a faulty graphics card and we got it replaced.

I got said graphics card back yesterday, put it back in and re installed the necessary driver and hey presto it worked! ...for a while. I started playing assassins creed and 15min into the game, my computer crashed. I decided to reformat my computer to ensure there isn't any undetected virus or software faults, but the freezes and random restarts continue to happen afterwards.

So I've used my computer today for about an hour, just browsing the internet and no problems. As soon as I run skype, it randomly restarts again.

I checked the hardware with HWMonitor and everything is running fine.

This is my build:

Corsair Vengeance CMZ8GX3M2A1600C9 8GB (2x4GB) DDR3

2 BitFenix Spectre 120mm Black Tinted Blue LED Fan

1 Arctic Cooling MX-4 Thermal Compound (4g

1 BitFenix Sleeved Molex to 3x3pin 7V Power Cable Black

1 Corsair GS-600 V2 Power Supply

2 BitFenix Spectre 120mm Black Tinted Green LED Fan

1 ASUS P8H77-M Motherboard

1 Intel Core i5 3570

1 BitFenix Shinobi Black Case with Window

1 Gigabyte GeForce GTX 660 OC with METRO

Not sure what else you guys need info wise.. im not very tech savvy. But anyone got any ideas?
From what I've read it could be a faulty power supply or issues with the RAM but I have no way of testing that without spending $$$ for some "pro" to diagnose it for me.
 
Update:
fresh reformat complete - all ok
hw monitor test complete - all ok
chkdsk complete - no errors
memtest complete - no errors

Issues:
Takes forever to load on start up now
Random restarts
Random freezes
** CD rom drive now stutters/makes clicking noises when CD inserted
** Games no longer start, I.e LoL/steam
 
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