Pardon my jumping in, here. It really sounds that you've got a defective alternator. It happens. As Tom suggested, you really should have it retested. There are different qualities of rebuilt units. Remember, you get what you pay for. We have switched to CarQuest reman. alternators and starters. They also have new ones available in increasing applications, but they can be considerably more money. And, it's not uncommon to get a whole lot number of bad units. Just because some el-cheapo parts stores have a lifetime warranty, that doesn't insure quality. Just that they'll replace it. GM alternators are about the easiest to open and replace the diode trio or regulator, but if it's still under warranty, you don't want to do that. Of course, make sure that all of your connections are clean and tight. And, that the battery is capable of accepting the charge. Again, as Tom mentioned, a defective battery can take out an alternator. And, discharging and re-charging the battery is really hard on it.