Might want to make sure that you have a solid corrosion free connection from the block to the ground. Sounds like it. Take off your passenger seat and take a look at the batteries and make sure that the connections are clean and tight. Right next to the batteries should be a shunt, if they are anything like the Military ones that shunt should be corrosion free as well.
Think of it like this. Turn on a flashlight and now turn it off. You have power going to the flashlight but no ground, so likewise before you snap diagnose check all of those connections. Bad connections make for a weaker starter. Check Voltage drop. You can check the voltage drop from the battery to the starter and droppage tells you either a corroded cable or bad terminal connections along that starter wire.
