If a system shuts down unexpectedly, it's usually a heat issue, but you could have also tripped a cutout circuit in the PSU. I'd have to double check, but I would think a 750W might be a bit on the low side for a 780 video card and everything else in the average computer, especially if you're working things pretty hard. It might be that the PSU you chose doesn't have sufficient amperage on the +12V rail to power the video card at full throttle. Every PSU has a certain degree of tolerance, usually +/- 5% on every power rail, but if you exceed that, it will trip a safety circuit designed to prevent an electrical fire.
The other possibility is heat. Just because CPU readings are at the high end of normal doesn't mean everything else inside the case isn't overheating. You also have to remember that there's currently no means of directly measuring CPU temp. What they do is stick a thermal sensor under the CPU socket and use that to estimate the temp of the CPU. You'd think AMD and Intel would have integrated some basic monitoring into modern CPUs to tell exactly what the temp and voltages are, but they haven't. But if your CPU is at 65C, odds are the ambient temp of the rest of the case is north of 60C and that can cause other components to overheat, such as the video card or even PSU.
So a good first test is to just remove the side cover from the system and see if the symptoms either go away or at least change after having done so. If yes, then you get the unenviable task of having to evaluate the thermals and air flow of your case. It's not just a matter of adding more fans. To do it well it actually requires some understanding of fluid dynamics as well as basic newtonian physics. Specifically Newton's third law about the equal but opposite reaction. Some people just throw in a bunch of fans aimed at blowing air out of a case, not taking into consideration that for every cubic centimeter of air you blow out of the case you have to pull in an additional cubic centimeter of air to replace it, otherwise you create a vacuum which is puts extra stress on the fans because they have to turn a little bit harder. All of which is just scratching the surface. So you need to think beyond just the CPU because the CPU is not the only part of the computer.
Putting the computer under stress (e.g. Higher end video rendering or gaming) causes the computer to suddenly shut down. It doesn't slow down before shutting down, doesn't restart or bluescreen, just goes black. Restarting the computer immediately after this happens causes it to shut back down shortly after loading the desktop.
From what I've read, this seem to be caused by the CPU overheating. I'm running an FX-8320 at around 35c when doing nothing, and going as high as 65c (According to AI Suite II. HWMonitor never seems to show over 60c) when gaming. I'm using the default heatsink that came with the CPU, and have already tried replacing the heat compound.
To get to the main question, is the default heatsink on the 8320 so utterly worthless that it can't keep the CPU temperatures safe, or could this be caused by something else? I already replaced my PSU, thinking that that might be at fault, so I'd like to get a few opinions before I start spending cash on more replacement parts again and end up replacing the entire PC. Again.
GPU: GTX 780
CPU: AMD FX-8320
Mobo: Asus M5A97 LE R2.0
RAM: A single Kingston 8Gb HyperX Blu, 1600MHz DDR3
PSU: Corsair 750W, CS750M
All parts excluding the RAM are brand spankin' new.
If the heatsink clearly is the problem, any suggestions for a replacement would be more than welcome. Preferably a little over what I actually need just to be safe. I'm not really sure what to look for in the sinks other than reviews, so any thoughts there are appreciated too.
I do plan on doing some overclocking. Not looking for the absolute maximum, just going to take it up a little