Arch's assumption was based on your fan's operation: they do react to higher system temps.
So let's have a peek at the temps with your computer temp monitoring program of choice (i regularly use realtemp). You might also want to peek at your bios' system auto shutdown threshold settings. For an i5 4670k, it might be at 75~80oC.
Is it summer in your region? Do you have an overclock? A hotter environ makes that stable overclock in cooler times, unstable. Hell, even without overclocks, some systems suddenly suffer from the hotter weather. These year's hot weather isn't like the past as well, I chalk it up to global warming.