Wouldn't the metal conduit act like an earth ground where ever it goes on the layout? Perhaps more so than say 14/2 flex wire? (fail TMCC works)
How are the wall outlets 3rd connection (earth ground) connected to the metal outlet box.
This may sound crazy but did you try turning off the florescent lights. Perhaps a leaky or noisy ballast.
I think we had another one of these mysteries a few years where the location was just tooo dry , no rain for weeks resulting in a poor earth ground.
