Crazy alternator issues?

Hi there. Well, the story starts when I was waiting for my daughter to get out of Kindergarten. I was parked with the car running, then it died. A neighbor jumped me, and I made it to an auto parts store and died right there again. They tested my battery and it said bad battery. So I bought a new battery. They tested my alternator and it said bad diodes. SO I took it to the mechanic shop to replace the alternator. The shop called me the next day and said the alternator is fine. I was shocked. Great honest mechanic, though! So, I drove it nervously for a few days just a few miles or so. Then today (about 5 days after the fact) I had to drive 40 miles and I was scared to drive it with my 4 little munchkins in the car. So I took it to an auto parts store who tested the alternator and said it was fine. I drove the 40 miles with the radio on for the first time since the first incident, and by the time I got to my destination, the battery warning light came on faintly after I started my car up to head home again. I took it to another auto parts store, and their machine said bad diodes again. What can be wrong? Is there a cable or some wire that could be deffective and not the entire alternator? Thank you so much for any help you can give.