[na2ub]

The alternator is more complex than you might think. I had a problematic alternator for years, before I had it rebuilt. I think there is even a circuit that runs to your alternator light, and if the bulb is out will cause problems charging. I wish I could be more specific, but if your battery is testing well, I would put my money on the alternator needing a new diode, or resister, or transister, or thermister or some other technical, electrical do-hicky.

I had mine rebuilt by a local alternator shop, but I think it cost close to just buying a new one.

Edit: I have the alternator with the built-in voltage regulator. Also, you should have a power draw, for, at least, the clock. Not sure if that is on the interior light circuit or not.