After you make sure your charging system is working right, and all your connections have been cleaned and recrimped, you might start looking at your battery.
I went through a battery every 3-6 months. I finally figured out that my problem wasn't the car, it was the battery. I had been buying them at Walmart. I finally had one of the clerks tell me that they fail all the time. I should have clued in on that when the last two times I went in, they didn't even bother to test the battery before offering me a new one.
I have since purchased an expensive spiral wound battery (Optima), and have been running without problems ever since (13 months and counting).