I would guess the alternator also, "however",
there is a part in the alternator called the voltage regulator that can fail. It's held on to the back of the alternator with 2 screws, easy to remove, inspect, and replace if neccesary. The regulator has a pair of carbon brushes which will wear out, causing this problem when eventually it dosn't contact the rings inside the alternator.
The voltage regulator is available seperately.
Nice thing is that the alternator doesn't have to be removed, the regulator can be easily removed without doing any additional work, and there is a ton of room on an SEL engine bay to do it.
Around here, I think the regulator is about a $50-60 part.
Or if you know the alternator is original and has alot of miles on it, a more expensive route is to put in a reman alternator, but I'd bet the regulator would fix it.