I believe it might be something in a recent firmware. I too had this occur, perfectly stable machine for days on end, then decided it wasn't happy. Would restart several times, then get stuck on 100% hardware until physically restarted. Then back to happy happy happy.
i havent updated the firmware
it was the one that came with the unit and it was hashing for 25hrs continuously at 1 point. I shut it down to add some fans or tried changing the temp threshold
the only things i changed was the rear fans, adding thermal paste to the heatsink and case.