Phone 5s is not working

Over the past two days, my phone has been acting odd. I have always had it in a case, and still am sure to not drop it. Two days ago, while it was in my pocket, it turned itself off. When I tried to turn in back on normally, it wouldn't respond. When I tried to hard reset it, it took nearly half a minute to start up, during which the display was also acting unusual. When I hard reset it, the screen brightened, as it usually does, and displayed the Apple logo. After about ten seconds, the phone shut off again for about a minute, during which the phone failed to respond to any input (chargers, button presses, etc). After the minute of no-responsiveness, it turned on, taking about two minutes. This had been happening every 3-4 hours up until yesterday afternoon, when it began acting normally again. When I took it to the Apple store, they said it was fine. About 6-7 hours after it started acting normally, it turned off again. When I tried to hard reset it, it took about ten minutes. I know that the home button and Touch ID work fine. I made sure there was no water in it, and no residual oils inside. This morning, after not shutting itself off overnight, it was working fine for a few hours, until I left for work. At work, it shut itself off, and I haven't been able to fix it. It never shut itself off when the display was active, it only did when it was asleep. While trying to force recover it, the display still would not work. I am wondering what the issue is, and if I need to get a new phone.