My first homebuilt system is failing, help

So I built myself a PC in spring and since then I've had so much issues with it. First off, my computer is BSODing after every day or two, the error codes indicate that I have a faulty RAM stick, but I have sent both of my RAM's back to the shop and they're telling that they're working just fine. Now the second issue is that my PC also restarts or shuts down randomly, when I built my system and booted it for the first time it was restarting after every hour or so, but then I turned my PSU the other way around (not that I had my PSU installed in the wrong way, but I guess it started overheating when it was blowing out the hot air under the case and it got too hot that way) after that it stopped restarting so often and now it sometimes doesn't restart or shut down for a week and sometimes it restarts multiple times a day. I have been told that I have a bad PSU and so on, but I'm thinking that if the PC BSODs too, then it might be some another issue. I've been struggling with this issue so much and I still don't know what's causing this.

System Specs:

CPU: AMD FX 8150 3,6Ghz
GPU: AMD Radeon HD 6950
MOBO: GIGABYTE 970A-D3
RAM: Kingston 2x4GB 1333Mhz
HDD: Hitatchi 1TB SATA3 7200rpm
PSU: Chieftec 600 14CS (600w)
Case: Thermaltake Commander MS-I