From my experience if you hear a "click"thats the solinoid engaging which means there is power geting to the solinoid but the internal contacts that supply power to the starter motor are dirty[coroded/worn]and you need a new solinoid,next time it happens if you bridge across the 2 large terminals on the solinoid[use a screwdriver] and the starter motor turns then your problem is the solinoid if it doesent,its the startermotor,could be a faulty armature