89 caprice battery voltage problem. Any suggestions or tips?

I have an 89 caprice and having problems with battery voltage going under 13 to 0. i just put a new battery and alternator thinking that was the problem, but it still does it. the voltage drops when i(use turn signals,use power windows, locks, seats, use brakes) stuff like that, it runs/drive for only like 10-30 minutes then dies on me, the only time the voltage goes back to 13 is when i'm getting a jump/charge, then it slowly goes down and dies. And i have checked/cleand all the ground wires and fuses i could see... at this point i dont know whats going on with it, so i'm looking tips and suggestions from a pro that know about this type of vehicle..Thanks!!