Had something like this up in Dublin in the bay area. Spent weeks filtering drops and chasing impulse from one end of the node to the other as soon as we thought we got it it came up on another leg seemed like we saw it coming from everywhere. We saw a bunch of power company trucks roll in the area and all of a sudden it dropped low enough to pass the node. Something on their grid was creeping on to our grid over the bonds I suppose.
Maybe try and disconnect any common grounds if thats even feasible.