1990 CAMRY DX-BIZZARE ELECTRICAL PROBLEM

I own a 1990 Camry with a 3 month old battery. The alternator is about a year old. Not long ago during a rain storm the car left me stranded. I took it to a WAL-MART Supercenter and they checked the battery and the whole electrical system and said it was fine. 3 weeks later the same thing happens. During this rain storm the alternator light would go on and stay lit for a few seconds then go off again. Then a jump start was required to get it to go. I took the battery back to the auto part store and they said it was fine and the alternator was fine. I don't get what's going on. Could the starter be shot?