Those voltmeter readings look fine to me. In fact, what you report is exaclty what I'd expect to see.

NOW, when I brought my newest (and third) Crossfire home, I noticed that my lights would flicker and fade at times. Before I could sneeze, that is, in a few days, the alternator failed completely. I'd suspect you are headed in the same direction - "tested fine" means little, other than it was fine right then. MINE was fine the first six days I had the car!