ok, every so often my car will be completely dead. i mean no dome light, no dash lights, and it wont even turn over, just as if i didnt have a battery in the damn thing. i checked my battery and its at 13volts. so thats not the problem. it will jump start, but while its running when i push on the brakes it basically dies. then everytime i jump it like that i shut it off and try to start it, and its completely dead again. then, a couple days later i'll go to start it and it'll start right up like no prob.whats the prob here?