[Cat Fuzz]

Bad grounds or other wiring, maybe. What you need to do is do a voltage test on the alternator while in the car. Check to voltage reading at the lug terminal on the back of the alternator with the engine running (good gawd be careful) and then check the voltage across the battery. They should be the same voltage within a tenth of a volt. If the battery voltage is significantly lower than the alternator output voltage you have wiring issues.

Also, the parts store alternator tester and your alternator voltage reading may or may not be adequate. If it's simply reading voltage output, that is only half the story. The alternator needs to be tested under load to see if it will put out the required current. You can do this while on the car by simply turning on a few accessories. If the voltage drops with accessories turned on, the alternator isn't keeping up. It could be putting out 14 volts but only a couple amps on the bench tester. A voltage reading under load will tell a better story.