Does it happen with different shore power outlets, different marinas? Our galvanic isolator test circuit (replaced last year for an isolator without the test circuit, due to problems with GFCI shore power) had shown a ground wire fault a couple times. One time they opened up the shore power pedestal and found a loose ground wire. I think maybe your boat is fine and the problem is with the pedestal. After replacing the isolator I bought a tester that you plug into the power pedestal to test for problems. I use it before plugging my cord in. Not all shore power outlets are trustworthy, believe me.