In daytime, while driving, it reads about 13-14v on the voltmeter. At idle (800 rpm) sometimes it's at 13-14v and sometimes drops to 12v or less. While driving last night at dusk, as soon as I turned the lights on, the volts dropped to barely 12v and when I pulled up to a stop sign the voltmeter dropped dramatically to 9-10v (red to yellow bands on the voltmeter) and the headlights dimmed correspondingly. If I reved up a bit it at least went up to about 11-12v and the lights got brighter. I have a Wilson alternator and underdrive pullys. Any idea what's wrong?