New GPU Troubles

I've recently just upgraded my GPU from a Asus AMD Radeon HD 7850 to a Gigabyte Nvidia GTX770 / 4GB and ever since my PC has been shutting down whenever i play a game or do something that will task the card. For example when i play a game that pushes the card like Arma 3 and Dayz it will completely shut down instantly with no warning and i have to disconnect the power and reconnect it to be able to turn it back on again.

It also does it when i try and do a system rating. I suspect it to be my PSU, it was only £20 so I'm guessing it's that because it was so cheap, also because i have to completely unplug the power lead it points to that. Also when i overclock my CPU past 3.8Ghz it instantly shuts down as well.

No components on my PC are getting to hot I've monitored it whilst playing these games for the short time i can and they all stay at fairly moderate temps.

GPU: Gigabyte NVIDIA GeForce GTX 770 4GB
CPU: AMD FX-8120 3.1Ghz (Stock CPU cooler)
Mother Board: Asus M5A97 EVO R2.0 Motherboard
Ram: Corsair Vengeance 8GB (2x4GB) DDR3 1600 Mhz
PSU: Artic Red 750W Power Supply

Asus DVD-RW Drive
X2 HDD's
Asus PCE-N15 Wireless adapter
X3 Case fans+Led's

Turtle Beach Headset X12
Logitech K260 Wireless keyboard and mouse reciever

Any help would be great! if you need any more info message me and i'll reply asap.

I wouldnt risk running a GTX 770 on a £20 PSU, it can actually do some damage to your components if its really bad quality. Regardless if the PSU is the problem or now I recommend getting a new one. The XFX550 is only about 45 quid, its a real solid well built PSU. Although if its not the issue then I have no clue.

I would say it is the PSU. 

Just a quick search found that it has terrible reviews. I would say there isn't enough amps on the 12V rails to power it or more likely the PCIE power as well as CPU power are pulling from the same rail causing it to shut down.