I have the same problem. Picked up Heart of Thorns the other day and it will shut the computer down and reboot randomly. Sometimes immediately as you click to play character, other times I get get maybe an hour and a half of play, and everything is set to lowest video setting or off. Nothing else I use or play does this and I can run ESO and Skyrim almost at max settings with no issues. Its not overheating, its not a RAM problem. It actually seemed more stable when I was playing before the whole game had finished downloading.
Tried the other fixes listed in this thread but no difference. Would just like to be able to play the game.
Win7 64, GTX560, 12GB RAM, SSD.