On a friend's 2000 Focus, I was having weird alternator problems like that. After swapping the alternator and still having the problem, I found one of the wires in the alternator harness was barely holding together. With the wire disconnected, the alternator would kick out up to 18 volts at times. Try pulling the wire wrap back a few inches from the connector and check the wires out. Also, Autozone does free alternator and battery testing on the car.