However, don't give up yet... if you've already 'cleaned the board', you may have removed subtle signs of damage due to corrosion. However, you may yet find telltale signs of 'oxidation/corrosion', however subtle, on vias and upper pads.
In particular, as per a post by another member battling his intermittent/ issues for over a year or more - he was told to look in the area of IC18 pin 11 of the digital board (HD14051 analog switch).
I did the same, and other than slight oxidation, the solder joint at the pin (component side) seemed okay, but a continuity test to the related track via (~ 1 inch away) showed otherwise - a complete open.
I think this particular area is a point of electrical attraction to the (ionic/charged?) battery vapor... The pcb annular rings are very small and so it doesn't take much corrosion to affect the pin/pad connection, particularly as there's not much copper to begin with.
So...check the topside component pads in this same area for any tell-tale dullness etc., use a continuity meter to verify the traces. Touch up as needed, if the solder doesn't take, it's a sign you need to attach a jumper wire, directly on the pin, back to a good point/via (not too far). Don't forget to resolder the bottom pads too.
Good Luck and post any question if you're not sure....