Seems odd the alternator would/could function properly unless I race the engine, or run +/- 1900 RPM (about 60 mph), triggering the battery light to come on, as well as the voltage gauge to drop very low.
I have had a number of alternators that would work quite fine under a certain RPM but go over that RPM and suddenly there is no charge, which sounds just like what yours is doing. A new or a good rebuilt one should fix you right up.
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