iPhone, turns off & on: black loading screen, help?

Hi, just a few weeks ago I bought a brand new iPhone 8 and just recently I have found that it has been playing up. Usually what would happen is that, unexpectedly a black screen with the spinning gear would appear: then turn off. A few seconds later it will turn on again and show up with the password login when you sign into your phone. This frequently happens, and it is very frustrating since I cannot access my phone without it happening. It’s also overheated sometimes. Please help!

Thank you.