Subaru electrical weirdness

I have a 1994 subaru legacy wagon. It runs very well, except that the engine electrical is weird: more than two years ago the radio, heater fan, and headlights started dimming in and out at random times. I cant find any pattern in when this happens (its not tied to RPMs or speed or driving time…). Then one night I was driving across eastern Montana and the headlights started getting dimmer and dimmer, and the engine started running rough and almost stalling out-- I got off the interstate and ended up rolling into a KOA campground just as the engine stalled for good. In the morning a guy from the camp ground helped me replace the alternator/voltage regulator (one unit). We charged up the battery. After that the subaru started up and ran fine–except that both the battery and brake warning lights (on the dash) were on permanently. I drove it thousands of miles that summer, but then moved to Scotland, so it sat since then for about a year and half with only very occasional usage by my dad. Now I am using it again: added a new battery, cleaned all the terminals and the ground connections. But the warning lights are still on, and the radio, fan, and lights still dim at random. We checked the voltage and its about 17, where it should be 14.5.

I assume something is wrong with the voltage regulator, but I dont understand why a voltage that is too high would cause the lights to dim. I don’t know if I should just replace the unit again (the voltage regulator can only be replaced with the complete alternator) assuming that the alternator I installed at the KOA was bad from the start-- or if I should search for some other electrical problem that is causing these issues