'03 ford Windstar electrical problem?

WE have a Ford Windstar and it has ran great until yesterday. As I was driving I noticed the ABS and brake light on. A little while later the battery light came on. When I was coming home I felt the car sort of giving out. When I make it home the battery was dead. We decided it was the alternator and battery. We replaced both and then it ran good today until a few minutes ago. Same thing happened only this time ALL the electrical stuff went out. Like the radio, signal lights, A/C, electric doors and so on... So I am trying to figure out what it could be. Any ideas? The car will start up great, it is just after it has been going and when you want to get more power?