Won't stay idling in park after start but once I drive it idle is fine.

Hello, this is not an alternator concern. You likely have an engine performance concern, possibly with too much air coming into the intake manifold when the engine is not yet at operating temperature. Look for an un-metered air leak, an air box leak, or some other related sensor or issue that would influence air getting into the intake. This is just a guess, and hands on testing is required to diagnose your concern.