Printer halted! Psu dead?
Reboots happen in 2 cases on a printer:

If the board power has been below a the operational threshold (~3 V) and the capacitors on the board are empty, the power dip can result in a reboot.
If a device connects to the serial-USB port, it reboots.

If you power up the printer not connected to any other device, it should run continuously. If it flickers and reboots, the power-lines to the board or the PSU are bad. Check those in reverse.
If it only reboots again and again when connected to a serial port (PC, Octoprint etc), your problem is the cable - it might be jiggling or be defective, or otherwise de- and re-connect again and again.