self exciting won't help. Have you tried tightening the belt or how about a new belt. Sounds to me when the alt gets a load the belt slips. Could simply be a worn out belt that is stretching to much. Start with the cheap stuff for fixing/troubleshooting. Something I always look at with an electrical problem is ground wires. They can do some funny stuff. Make sure they are all tight and clean connections. Have you tested the battery? Voltage is 14 unless headlights are on. I assume you are looking at the gauge in the dash. Could be that your battery is already on its way out and the alternator is working like hell just to keep it at 14 volts then when you put the headlights on it can't keep up. Just some things to check. Good luck.
