Weird Electrical Charging Problem

I think I've got a tricky one on my hands. My daughter has a 97 Honda Civic HX. Thanksgiving night (of course) she called to say her car wouldn’t start. I drove over to her house and jumpstarted her car. The headlights were repeatedly flashing brighter dimmer brighter dimmer. I checked the battery voltage with my meter and it was jumping from below 12 to nearly 14 volts so fast the meter couldn’t keep up. My immediate thought is that the internal regulator was freaking out. I left the battery charger on so she could get to work in the morning. The next day I swapped cars with her and took the Civic home. I removed the alternator and took it to Advance auto parts. Since I didn’t want to waste my daughter’s limited funds, I asked them to check the alternator on their machine. The machine said it was working fine. I put it back on the car and checked the voltage at the battery – it was approximately 11.5 volts, i.E. Not charging. I drove the car straight to Advance and asked them to check the charging system on the car. They brought out their machine and checked it and said everything was fine. I checked the battery voltage with my meter and sure enough I got about 14 volts. I swore to the guy that only ten minutes earlier the battery was not charging. He just kind of looked at me funny. I gave the car back to my daughter and told to wait and see what happens. It worked fine until the Friday before Christmas; same problem again. This time I just bought a new alternator and put it in. Seemed to work fine, 14 volts at the battery with the engine running. Problem solved, right? Wrong. Last night she calls me to say it wouldn’t start again. I jump start it and the lights are doing the bright dim bright dim thing again. Check the voltage at the battery and I’m only getting about 11.5 volts. Race the engine and the flickering stops and the voltage drops to around 8. What am I missing here? I used to be a mechanic, and I’ve replaced hundreds of alternators over the years. Generally they either work or don’t work. Put a new one on and you’re good to go. Sometimes a bad connection can cause a weird problem, but I don’t think that’s the case her. Any ideas?