|Have you tried to boot into the BIOS (setup) screens and see if the unit is stable there?|
May be premature to replace the motherboard at this time.
Overheating can cause rebooting. When you disassembled the unit did you remove the heat sink/ fan and clean off all thermal pad/ paste, then apply new paste according to specs for your processor?
See the link below for proper application method. Use the Celeron mobile method. Any brand of thermal paste will do.