348 Alternator Problem

I notice when my 348 starting up cold, at idle, the alternator will charge the battery normally. After the car warm up, at idle, the alternator seems not charging the batter as I use volt meter to test. If I give it some rev, the alternator seems to be charging again. The alternator seems working normally if I drive the car on the street. It just seems it will stop charging when the car is hot and at idle. Is my idle RPM too low? What seems to be the problem?