iphone 6se shuts down repeatedly

The phone started shutting down several times per day and requiring a forced restart to turn on again. I have restored the phone using a backup- that did not work. I restored it as a new phone-- better for a couple of days then the issue began again. It is currently shutting itself of many times per day. The battery is charged and appears to be working correctly. The system says its at 91% efficiency. Running IOS 11.3 which I updated after the problem started in an attempt to fix. I'm ready to throw this thing off a tall building. Any ideas?