Default Nexus 7 automatically shut down after a few hours

Hi guys, i just got a new Nexus 7 32gb wifi only with 4.3 stock rom. It's fine for a first week use. But yesterday, I came home realizing it'd shut down, so i had to press power button for 10s and then it's turn on. But after a few hours not using , it turned off again. I google for it and somebody make some suggestion, like stay away from my cover ( but sadly i don't have a cover) ,factory reset ( i did but it's still remain), use Safe Charge app to keep it from suffering from sleep of death ( but mine shuts down while not charging). Any ideas what this issue is?