2001 Infiniti I30 Problems Starting after warm

its probably the starter motor and the added juice your giving it is enough to engage the solenoid. the way to check it is to make sure you still have 12 volts at starter fat wire when cranking and also make sure the smaller wire stays hot when you are on crank position, but i think you will find that its a bad starter motor. when they made these starters they used a thin round disc of copper in the solenoid that rotates and is supposed to make good electrical contact when the solenoid is engaged but this piece gets arc marks on it and occassionally will hit those arc marks and not make a good enough connection.