I have a 2007 Sig 330 with merc 8.1 Mags. The port battery intermitenly will not crank the engine. Batteries are isolated one for each engine plus the house battery. All 3 have been replaced, but port still fails. Load test was done and passed. Alternators are charging when running. Battery charges indicates they are working fine. One minute the battery seems dead ( But will start with the parall switch depresed while starting. Can be "dead" for days then next time starts without any problem. Any ideas?<br /><br />-almost there