"If you don't find a loose
"If you don't find a loose connection per Jay, check for AC voltage at the DC voltage output.
Use a digital voltmeter on the AC scale and attach the + lead to the alternator's DC output terminal and the - lead to the alternator case. If the meter displays more than 0.4 AC volts, one of the diodes is shorting out and needs to be replaced. Erratic AC voltage can make a DC analog meter jump around.
Fix minor things (replace wear items for preventative maintenance.) that cause major problems before they break.