Have you checked your bonding system lately?

[smalldog]

Dont forget to check your bonding system. Especially if you leave your boat in the water and have shore power.
Check all the connections for corrosion and insure you have less than 1 ohm of resistance between your thru hulls and the negative post on the battery.
If you have a clamp meter that will do millivolts you can clamp the shore power cord to check for AC leakage also.