I can not give you any advice. The symptoms are too vague and without seeing you it would be one big guess. I have no idea what is going on with you, as far as your wife is concerned, I hope the dentist pulped tested the tooth to check the vitality o9f the nerve.
silver amalgam fillings corrode over time and actually expand in teeth, creating micro or sometimes macro fractures. i doubt lemon juice is causing pain in a tooth that already had root canal--- especially adfter so many years of being fine. it is probably another tooth with root exposure, decay, a fracture, or an old filling that has simply overstayed its welcome. not all dental problems are diagnosable on an xray.
In my own personal case, I only feel it if I bite down too hard on the molar. It's not really pain, just sensitivity.
could be a fracture in the tooth, which, again, may not be visible on a film---