Not sure I agree with the under-powered post. 650w should be enough for that build under stress as it's rated at 521w (loaded) and 20% puts you at 650w. Since you were downloading a game (ie, not stressing the gpus/cpu) I'd think it was highly unlikely that a lack of power was causing this.
That said, even good PSU (Xseries is a great psu) can be bad. I'd suspect you either have a bad psu, or something else is at work (thermal paste application, or mobo issue, for instance).
You're right, I added 2 570s on accident. My mistake.