[quote]The keyboard and top half of the screen functioned normally for months.[/quote]
'''Did you ever clean the board''', remove and clean under it?
If not you probably have corrosion on the board or connectors. Look at it. Try cleaning the board with an electrical contact cleaner & tooth brush - followed by 91% alcohol.
''' ''LET DRY TWO DAYS.'' '''
reassemble.
Good Luck,
N.