Do you get a BSOD when the system fails or does it simply power down without any indication or warning? Also, by checking your drives I assume you have done extended tests to see the SMART status, in which case you could post a screenshot of it here :). Something simple to check out would be to make sure all your cables and components are firmly in place. Do a malware scan if haven't done so already. You could also re-seat all your components. If none of that helps I would suggest “stripping” your system to the essentials (Motherboard, CPU, one stick of RAM (graphics card if you do not have integrated)) and see if the problem persists. Another thing to test would be your PSU. P.S. Could you tell us if you can here any beeping sounds from your motherboard? Those are codes and they could point us in the right direction. Another thing that would be helpful would if you could provide information as to what your temperatures are.
Keep me posted, Frost_WD