Is the cold water pipe cu? Do you have good ground rods? Connections. Sounds like you have a bad trans from the power co. if its happening w the main off. and your grounding of the service is not doing its job very well or is doing the whole job and your plumber friend found a little better path to ground. I just finished with a similar problem. The cold water clamp was loose and customer noticed it was arcing and his lights were blinking/flickering. I tightend it and the problem went away. After many calls and visit with the POCO they decided it was a bad trans. It seems the neighbor had a problem too and didn't know it. He paid another sparky weeks before to change his panel LOL.
Ob
A bad transformer does not cause elevated ground voltage. As for tightening up the the ground water clamp your just masking a dangerous hazard called an open service neutral. The op should have the poco do a double check that everything is ok on there end and also try to make an equal potential grid to eleviate the symptoms.
