When you jump start the car (you say that works) are you connecting both cables to the battery, or are you connecting the negative to chassis ground. If the former, I'd check the negative cable from the battery to the chassis for a good ground. If the latter, I would make sure there is good contact between the cables and the battery terminals. Check the voltage across the battery terminals when car is running, then turn the car off and check again. What's the difference between the two readings?