Car has 150Kmiles. Has a big Bosch alternator apparently standard on Turbos.

This is more of a story that started back in December (see below). Bottom line is that the lights dimmed out on evening commute - as if the alternator was not charging and I was running off the battery. After a short rest the car restarted and the lights were normal. I found a plastic plug in the engine compartment while inspecting but couldn't tell where it came from. It's edges were melted.

The car was in a body shop over Christmas, but did the same trick again this past week.

I took the alternator out for testing and noted that there were what looked like carbon streaks radiating out from the center on the front of the pulley. I realized that the melted plug looked to have been a plastic cover for the nut that fastens the pulley to the shaft. Is this right; is there a plastic cover over this nut? If so, why would it melt?

The alternator produced a constant 15V on the test stand. The tech told me that a 150Kmi alternator probably has worn out the brushes, which would explain why it sometimes charges and sometimes does not. But he also said that the 15V output indicated a problem w/the regulator.

I'm assuming I need another alternator, and am considering either putting a regulator and brush kit into it (dealer price: $153), or replacing it w/a rebuilt, whatever is cheaper. But I am concerned about the melted plug, and some symptom I may not be correctly diagnosing.

Any thoughts?

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December's message:

B230T w/150Kmis. When I started the car at work tonight there was a squeal coming from the engine - sounded like a slipping belt, or power steering squeal. I thought it would go away as the engine warmed (27 deg here). It did, but recurred a few times. Before I got home the headlights and instrument panel began to dim. I got home just before they went out completely. I opened the hood w/engine idling expecting a broken alternator belt and frozen alternator, but everything looked OK. After letting the car idle a few minutes I turned the headlights back on and they were bright, as was the panel. I turned the car off and restarted it w/o trouble.

What has happened? I obviously don't have confidence in commuting in this car until I sort this out ...

Thanks.