1985 Oldsmobile Delta 88 dim alernator light

I got this car from my mother and it had a dim alternator light at idle. I replaced the alternator with a rebuilt one and problem solved. Now about 6 months later and about 1000 miles the light burns dim at idle and sometimes will go out at highway speed but not always. I tested the output with the battery disconnected and engine running and got 15.3 volts. How can I test the amps output to see if it is outputting enough current or would that be necessary? Can it put out enough volts and not enough current and cause this problem?