I agree that the motherboard is likely to be the fault here. However, if you want to try a few things the first thing I would do is re-seat all the components into the motherboard.
Unplug the power supply cable first!!
Then one by one take out everything that is attached to the motherboard and replace it again. Doing this can rejuvinate some connections and obviously costs nothing more than your time. just be careful to "ground" yourself on the machine by trying to keep your body in contact with a bare piece of metal on the case. This will prevent any static discharge (even better if you have an antistatic wrist strap).
Good Luck!