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Sounds like your battery / starter / connections are fine which leaves the alternator. Take it out and take it to an automotive electrical shop (not Autozone, Pepboys, etc) – look in your yellow pages under automotive-electrical. They will (or should) test the alternator at no cost. If it needs some work they can rebuild it for about 1/3 or 1/2 of what a new one would cost.

 

Battery voltage steadily going down with engine running (at idle) and all electric bells and whistles active: alternator is not delivering enough juice to compensate for the load.
Current of 60 amps @12V is about 700 watts: I think your alternator is rated to deliver between 80 and 90 amps maximum, 60 is too low a value.
Is the alternator belt tension within specs?

 

Thank you for checking the belt tension. Exactly how much belt deflection did you get when applying 10kg (22lb) force between pulleys?

 

I feel for you as I recently had a problem with the alt light on my son’s car, also a 91 that drove me up the wall! My symptoms were a little bit different, as I understand yours, but for what it's worth here's what I went through:

The problem was that alt light would intermittently come on when the car was first started when the engine was cold. If the light came on sometimes you could just shut the car off, restart it and the light would go off. Sometimes you had to restart multiple times before the light went off. Other times restarting didn’t work at all but if you let it sit for 15 to 20 minutes then restarted then the light would go off. The strange thing was that once the alt light went off you were good to go for the rest of the day and the problem did not appear again until the next morning. I removed the battery and alternator and had them tested at NAPA, and to my surprise, they both were good. Along the way, I did all the usual voltage and resistance checks, many times, and confirmed that when the alt light was on the alternator was indeed not charging and when it was off it was. I checked the grounds both to the body and to the block and they appeared to be good. I was about ready to give up on it because I didn’t know what else to do. I then recalled something about conductivity/resistance being affected by temperature so I decided to physically check the battery grounds. I removed the bolt that held the short/small black wire to the body that came from the negative battery terminal. The connection looked good from the outside but once removed it appear to be slightly corroded on the inside. I didn’t think that could be my problem because, as I said before, I had already checked for that possibility. I took some steel wool and cleaned up the connection and like magic the alt light problem went away.

I don’t completely understand it, but that appears to have solved my problem!

 

That's funny, just this morning I was thinking of this post and was remembering some electrical issues I had with a '95 Camry. The issue was that the car's electrical system would just cut out, about once a week or so, when the car was sitting but it wasn't the starter.

I ended up just taking the shotgun approach - I replaced the negative terminal cable, the positive terminal clamp, the battery (it was about four years old), and checked the positive cable connections to the car (the ground) as well as to the starter. The problem went away.

With the age of your car, just replace the negative terminal cable, I think the Toyota OEM cable was about $20. It just wrapped under the battery and connected to the car right under the battery. And if your battery is more than a few years old, just replace it.