I don't have an answer but here is my advice because you say "any" is welcome lol.

I'm a licensed electrical contractor and my company specializes in commercial energy efficiency projects...im NOT a master electrician....that being said, we encounter lots of electrical gremlins when retrofitting old buildings and upgrading theatrical lighting systems.....

as you stated replacing the ground is a good place to start. Then its simply a process of elimination. you probably already know this, but you need someone to push you into diving in and committing. Take one switch or item out of the equation, and check to see if there is an impact. If that does not work, put it back and try another. So many times my guys have said that the problem was not X, and when I make them test it, that turns out to be the problem.

anyways, that is my .02