Camry :: Batteries Replaced Several Times, Keeps Failing

My wife drives a 2005 Solara 4 cylinder 2AZ-FE. Over the past few years, it has been going through batteries about every year and a half. The batteries have all failed on the load test.

So far, I've removed the alternator and taken it in to get it bench tested, tested fine. I had the full charging system check with the machine, starter tested fine, alternator volts were fine, only problem was the CCA load test (for which the battery was replaced under warranty). I tested for parasitic battery drain several times, got some drain for awhile after car is powered off, but it eventually falls to zero. I checked for voltage drop across the wiring, visually inspected the wiring to the starter, alternator, etc. and made sure the ground connections were tight from the battery to the frame and engine. Everything looked good.

The only thing I can come up with is that the most recent battery boiled over a bit at one point in time because there was evidence of acid spillage. Also, once (and only once), my wife noticed her lights flickering while her car was idling for awhile during a cold morning. These two things lead me to think it must be the alternator (voltage regulator), but neither the bench test nor anytime I test it at home reveals any issue.

Maybe it's an intermittent problem from vibration, etc. I just hate to plunk down the cash for a new alternator if that isn't the issue.