Hello, I recently built myself a decent gaming computer with an asus p8z77-v lx motherboard, 16gb (4x4gb sticks) of ram, an Intel Sandybridge i7-2600K Unlocked Core i7 Quad-Core Processor (3.40GHz, 8MB Cache, Socket 1155), VTX3D AMD Radeon HD 5450 Graphic Card, running from a Powercool Modular 750W ATX Computer PSU, and a 1tb hdd (can't remember the brand) I only have a usb keyboard and mouse running from it but it will just randomly turn off, despite working fine for a few days, now it won't run fro 5 minuets without shutting off. I got all the components brand new and even replaced the motherboard after it stopped booting up after running perfectly well the night before
I'm not familiar with that brand of PSU, so I can't say if it is a trustworthy brand or not. But usually, shutdowns and/or reboots are a sign of the PSU not being able to maintain the necessary votage(s). Or simply being defective.
Btw, that is a nice rig... except for the w-e-a-k graphics card. HD 5450? Why did you skimp on the most important part of a gaming rig???!