"...started fluctuating from about 12 to 14 volts every 30 seconds or so..."

Maybe the AC cycling on 'n off?

1) Have it done professionally so they can check the output using a bench tester. If you know a rebuilder, have yours rebuilt. Be sure to mention the unusual cycling so they'll look for it.

2) Sometimes with, sometimes without -- depends on the rebuilder. You can zip the old pulley off in 2 seconds using an air gun.

I can speak from personal experience for Kore-Tek (4060 Acushnet Ave, New Bedford, MA 02745, 800-258-0940, 508-995-3755, owner Mark LeCompte). Mark does a first-class job rebuilding alternators and starters for European cars, Volvos first on his list (he's a brickhead).
--
Don Foster (near Cape Cod, MA)