As was already mentioned, a spontaneous shutdown is almost always a power issue.
Your power supply exceeds the needs for your system, (but nice to have if you ever get a gpu).
So, I would guess that you left off the cable next to the apu from the psu. Or the cable is bad, or
not seated properly.
Also, the apu only uses a max of 65 watts, and there's no added gpu so I wouldn't think temps would
be a problem, but, if the cpu cooler isn't mounted properly, it could be. You can check your temps with
a program like HWinfo. Also, if temps were the issue, the apu would throttle itself, not shut down.
As others have already said, make sure your memory timings and any overclocking is sorted out.
Good luck!