It looks like I'm not out of the woods yet.

When the car is completely turned off, the voltage is 12.52 (I measured the new battery last night as 12.7 at Pep Boys) At idle, it dipped to 12.4 and when I held the throttle spool, it came up to 13.9. I then turned on the headlight high beams and ran max AC on 4. At idle, the voltage was 11.8 and with the throttle, 13.5. Every time the AC compressor clicked, it dropped to 12.9 volts or so.

Because the voltage increases with RPM, wouldn't this mean that the alternator's doing something? Could this be a case of the alternator kind of works, but isn't putting out as many amps as it once did? It has a SpecialTAuto sticker on it, but I can't discern whether it's supposed to be a 120 amp or 140 amp alternator. When Melissa took the alternator to be tested, the guy only replied that it works - he didn't give her a printout that describes the amperage rating (I'm not sure if that's common practice or not).