Sounds like a bad voltage regulator. When was the last time this was checked? Or if a solid state type, just changed out?
Also check - the batter cables - are they fairly new and clean, and properly tightened.
Ground connections - properly grounded, clean of dirt and grim, and a tight connection.
Main cable down to the starter solenoid - is this fairly new, end connection clean, and properly tightened nuts at the solenoid.
Also check the starter solenoid connections, threaded studs, and nuts.
Lastly, did they hook up the wire to the alternator correctly and what do the end connectors look like? Nice and clean? Dirty or a DIY wire connectors and crimp job?