I agree DJK, sounds like a loose neutral on the PoCo's breaker, or maybe intermittent contacts on it. I am assuming their breaker is a standard type, non-rcd, thus it wouldn't mind the loss of the neutral, and maybe the disturbance caused by switching, testing etc is intermittently connecting and disconnecting whatever fault it is. Very mysterious, and lost neutrals always make me highly nervous, people assume no power=dead.
