Computer shuts down then restarts when playing

[buzz_lightyear]

Hi, I recently changed the graphics card. I had a 4870, and replaced it with a 7850. I had very frequent freezes with games, so I RMAd the card. The new one had the same problem. I obtained a refund for the graphics card and ordered a GTX 660 (which has similar power requirements). In the meantime, I plugged in my 4870 again and the system was very stable.

Now, with the GTX 660, the computer shuts down randomly when playing games, and then restarts after 5 seconds or so. The shut down doesn't happen immediately and usually after an hour.

Here are my specs:

CPU: Intel i5-2500
MB: Asus P8P67
RAM: 8 GB OCZ DDR3-1333 CL7 (running at CL9 as per SPD specs)
VGA: ZOTAC GTX 660
PSU: Nox Urano 500 W (replacement for a Enermax Liberty which died a couple of years ago)

None of the components is overclocked. I checked the temperatures, and the CPU doesn't get hotter than 70 °C, while the GPU is between 60 and 65 °C. I stress tested the CPU and the system is stable.

Are my problems caused by a faulty PSU? Faulty Motherboard? Which one is more likely? Why doesn't the problem occur with the 4870 (which has higher TDP)? Have I been extremely unlucky and three out of three graphics card I bought were defective?

 

[mi1ez]

Did you properly get rid of old drivers or just install the new ones?

 

[buzz_lightyear]

You are right I forgot to mention the drivers. I completely removed the AMD drivers before installing the gtx.

 

[buzz_lightyear]

Which software may I use to log the voltages to file?

 

[lostgamer_03]

Is there a strange smell whenever the PC shuts down? If so, then it's most likely the PSU.

 

[buzz_lightyear]

I haven't noticed anything. I'll try and smell the PSU to make sure!