You mentioned that the issue is intermittent, and that the current prognosis is to replace the mainboard - which I tend to agree with, since you tried pretty much what members here and yourself suggested so far.
Acer has issue with certain models, like any other brands. Luck of the straw, for many the same model works

My recommendation is the charlatan approach of "baking" the board and hope that it spreads the "warmth" evenly, and cross the fingers.
cheers ...