Why would your Bonneville charge somedays and then not charge other days you replaced the alt and battery?

I have a 92 Pontiac and also had similar situation. Sometimes the voltmeter would go to 14volts and other times it would sit at 12volts. When I tested output from alternator to the battery with voltmeter, I did not get a good reading. Upon removal of the alternator to get it replaced, I noticed that the smaller (red) cable that goes from the alternator to the battery was loose in the connector itself. I cut the cable at the connector, stripped the cable some and crimped on a new connector. Reattached to alternator and put alternator back into vehicle. All has been fine since. Hope this may fix your problem.