I had the similar problem with my 93 SC300 with about 90 K. Headlights dimming, stereo resetting occasionally, strange sequences of lights on the dash (ABS, battery etc.) while using the turn signal (particularly the left one). This problem was intermittent until the car ultimately died with a dead battery. Took it to Lexus dealer, they replaced the alternator and the symptoms improved but were still present. I took it back a month later and they told me that the alternator controller unit was bad. They said this part rarely fails and they don't routinely replace it when they do the alternator. They were decent and only charged me for the part as they felt they had not diagnosed the problem correctly initially. This solved the problem completely - however 1 year later the same symptoms are showing again.
I'm very interested in what you find out - please post your results. I'll probably take it back to the shop so I'll post my results as well. I can't imagine what explains the controller unit (I can't remember what the part is called) could "fix" the problem but then fail again 1 year later. Could there be some other electrical fault (bad cable, bad grounding) that is causing the controller to malfunction or fail?