Samsung Galaxy Note 2 keeps rebooting

I got a Samsung Galaxy Note 2 now for more than 2 years. Since a week my phone reboots without an error. And when it reboots then it will keep rebooting, my phone can´t startup anymore. I need to remove the battery and charge it a little (even though my battery is 50% full) before it can startup normally. I already tried to factory reset my phone but this didn't help. Is there a way to determine why my phone keeps doing this or do I need to accept that my phone is broken?