Pc freezes when running gpu intensive tasks.

This is my first time on this forum so if I seem rude or stupid i'm sorry.

Let me tell you a story: I bought a second handed pc (yes the risks ) for 280€ and it featured: i5 3470, 8gb GEIL ddr3 1066Mhz ram, asrock h61m, gt420, corsair vs450 psu, 750gb 7200rpm hdd.
It was all built in an medion case wich he took al the parts out and put the listed above back in.
So i wanted to use it for gaming and put an msi gtx 750ti 2gb oc in. 


So it ran fine for 3+ months. Then i force-closed it (hold the power button in) because it didnt want to shut down, the next day i booted it up everything ran fine but windows said my hard drive needed an check so it did and rebooted, it asked again but this time when it rebooted windows crashed. 
I installed an new copy of windows 10 and the drivers with it. 
Everything looked to be fine until i booted up gta V and it crashed after 15 minutes, displaying nvidia driver stopped working and has recovered, i found a 'fix' online but it still didnt work.
The next day i took the side panel off and it ran fine for 45 minutes and then it crashed again.
So I took msi afterburner and furmark to check the temps, it was on 58℃ and crashed.
So i thought it couldnt be the gpu temps.
So i dont know what it is, the psu, cpu, ram maybe even the motherboard.
Any suggestions/fixes would make me very happy!
I wish you an nice day. 

I have checked the voltages on my PSU, i let furmark run until it displayed nvidia driver has stopped and has recovered. It also states the temps on my gpu.

http://prntscr.com/9rb9mc

Can't be sure it's the same issue but I've just reverted to Win 8.1 from Win 10 in the last week or so due to persistent graphic driver failures that crashed the system at times. Most of the time it would just freeze the screen. Eith wayt it was unacceptable. Nothing I did fixed it, including reverting drivers or reducing game settings etc. I would at least consider it a suspect.

I've already re-downloaded my driver. Its not windows as it ran fine before, I think its an hardware related issue. Could the psu be it? It got as low as 11,458 volts. Maybe this psu is better https://www.afuture.nl/productview.php?productID=1106979.

@Captain_Chicken I would like to be assured its fixed if i buy a new one. I tried it with the psu out of the case and it froze up (psu out of the case for more cooling). Here take a look: http://we.tl/qcTbSHs5Yd

That link shortcuts to a download. I do not download anything to my pc unless I know exactly what it is. If it is a youtube video, just give us a link.

Edited by TimBC, 19 January 2016 - 02:02 AM.