Just this week the alternator on my van went out. When the battery light first illuminated, I tested the battery and while running, I was getting 14.5v at the battery, indicating that the alternator was doing okay. I cleaned the corrosion off the posts, started it up, and the light went away. The next morning it came back on again. I drove up to to Cheyenne for work, sat in my van all day charging stuff and draining the battery. Then on the way home, once the voltage dropped enough, lights started coming on and the trans decided it only wanted to be in 2nd gear.
So I guess the point was that at first it tested okay, then the next day quit.
I would check all of the grounds you can find. How are your motor mounts? Is it possible that at high RPM the motor twists enough to loosen a ground?