A very common fault on motherboards is on defect electrolytic capacitors.
In general the 1000uF capacitors used in power line filtering are dryied out or almost ready to blow.
By replacing them, the motherboard is functional again.
Common faults for this problem are, random resets, not proper power-up or crashes.
I fully agree with pl4tonas, the most common reason for a failing motherboard is the electrolytic capacitors, normally the electrolytic capacitor close to the CPU and PSU fail earliest, but also other electrolytics on the motherboard use to fail.
Another thing to bear in mind when the electrolytic liquid comes out it can short or disturb some functions if it comes on the motherboard PCB, so make sure to clean thoroughly!
I have repaired motherboards with success and it's 95% a matter of just changing the electrolytic capacitors.