I have a 1998 Subaru Forester with about 100k miles. Over the past week or so, from time to time, the battery, brake, and ABS warning lights would come on for a second and then turn off. My first suspicion was a loose cable until I was on a long drive last night.
This same problem happened about 20 times over the course of 2 hours, and what I noticed was all lights in the car would get bright, then the battery light would come on, followed by the brake warning light, and then the ABS light. Then the headlights would go back to normal brightness and the warning lights go off. I now suspect the alternator, but I just had it replaced about 11 months ago. It seems odd to me that an alternator wouldn’t last even a year.
You are correct about the alternator as being the source of the trouble. If the current one isn’t warranteed I would replace it with one that has a lifetime warranty.