Wel, it shouldn’t reboot by itself, maybe logging shows a reason. You could switch hardware; pi or sd card.
You could run with default magicmirror config. No extra modules installed. To see if its still reboots, and after that slowly add your extra modules to check if one module causes the reboot.
When running in a frame, enable ssh, so you can remotely acces the pi. You can also use pm2. This process monitors your magicmirror en restarts the magicmirror when it stops or after a reboot.