Abnormal shutdowns during use and not

@riccardorusso start your phone is safe mode and see if that does it in that mode as well. If not then the issue can be a bad app. You will have start remove those until you find out which one caused this.

1. To activate Safe Mode, hold the power key until the Power off icon appears.
2. Press and hold the Power off icon until the Safe Mode prompt appears.
3. Select Safe Mode to confirm.

If it continues to shutdown, it is possibly a power issue, so you need to consider checking the charging board for short circuits, missing or burned components etc. Also consider replacing the battery.

Since iFixit does not yet have a guide for that, use something like this video to work on your phone.

IF nothing helps, I would also suggest that you backup all of your files on your phone to your computer and then do a factory reset. Remember, with that you will lose all of your saved files, password etc. Make sure you have proper backup on that.

Here is how to do a factory reset according to Samsung