Double check your ground. Does it start if you jump the two large terminals on the solenoid with a screwdriver? When you push the button does it click or rattle? It sounds like something isnt getting full voltage without an extra battery. Hook a volt meter to the starter terminal and push the button. See if its getting voltage down there if there is none trace it back. If its full voltages its something in the starter, if there is a voltage drop its the solenoid.