it could be your PSU, whats the exact model? Are the shutdowns happening whilst gaming? If so, run something like HWmon and watch the temps as you game, take a screenshot when you do. Then we can judge better whats going on.
Also your system is not balanced at all. You need more ram if this machine is for gaming. 4gbs is way too low.
You have a savage mobo, a low-mid CPU, paltry amount of ram, and a decent 1080p GPU.
You could get 8-16gb of fast ram. A better CPU and a better PSU. Then you'd call it a good gaming machine.
If you are OC'ing CPU or GPU, then try both at stock and see if the problem continues.