Have you tried running a Capacitor? I had the same issue running 4 amps to the point the boat would shut down. As well I changed to running one yellow top optima(for stereo systems) and one blue top marine optima batterie , changed from 4ga wire to 0ga on my larger amps and every time I pull my boat out I trickle charge both batteries. Make sure everything is grounded to your batteries as well.