could be a fracture in the tooth, which, again, may not be visible on a film---
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.
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.
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.