Hi,
Make a visual inspection and look for any sign of humidity in the motherboard.
Some times it will be more expensive the labor of investing more than 2 hours looking for the root cause than the repair itself.
My advice, whole cleaning of the motherboard and if possible ultrasonic treatment to remove any sulfation in the components.
If required apply a good flux and heat (heath gun mid power) 2.5 mins p/side.
Good luck and good repair.
JM