Windows 7 based custom PC reboots unexpected

Hello, Im new to theese forums, but I bumbed in here while driving down on the google highway, searching answers for my problem.

So, here is the story of my past week.
I decided to update my old computer a bit and ended up buying a SSD for OS, new mobo, new DDR and a new case.
I did a fresh install of Windows 7 Home Premium 64Bit on the SSD and formatted the old partition that was for the Windows installation.
There we no errors during the install.
Now during gaming or running BOINC, my PC just reboots, no BSOD or anything, like complete powercut and restart.
This leaves no error message to the event messages, except for an error that Windows was shut down unexpected.
Ive tried a whole lot of tips and tricks.
One interesting thing was that the computer had restarted while running memtest86+ in Windows, I wasnt at my computer while it had happened, so I do not know if it said anything.
Ive completed Disk Check before Windows boot, Ive done sfc /scannow and it completed 100%.
Ive upgraded my BIOS to the ASUS Latest, NVidia drivers updated aswell, ive returned the stocksettings for BIOS, made sure that there is nothing there to OC it automatically.
Seems like something happens during load usually.
Atm my computer is running memtest86+ v4.2 USB Boot, gonna make sure if its the memory problem.
During the test, Id like to hear if anyone has any ideas on what to test next if the memtest doesnt bring up any problems.
Thanks beforehand,

- H

1h and 1 pass later, gonna keep it on but doesnt look like memory is the issue here.

1h 15min in, 41% of test round 2 over and it froze up. Test was 4-6Gb, so removed the memory from block 2 and gonna start testing on 1st memory only, if its ok, Ill switch to the second memory and keep testing that.