[jeffcoslacker]

The part that kicks the pinion gear in the starter out to engage the flywheel gear is operated by an electrical solenoid, which uses an electromagnetic field when voltage is applied to make the thing move. When the windings get weak or broken, the field is not as strong, and the solenoid's action gets weak. Applying more amperage (charger hooked up) will let the remaining windings produce more magnetic power, but also overtax them to death eventually.

You need a new starter.

It sounds as if the solenoid is the actual problem, but it is part of the starter, so if it's windings are going bad, the actual starter motor windings won't be far behind. They don't cost that much.