If you are seeing it on the amp meter and on a seperate volt meter I would look for a bad rectifier diode in the alternator. Sounds like the diode is intermitant and acts up when it's warmed up.
As I thought about it this weekend I was thinking maybe it could be a leaky or intermittent diode causing it. Since I haven't been able to find a battery to borrow I'll check out the alternator diode pack pack. It's a rebuilt alt. and seems like they just don't last like they used too.