Starter problems of the 2004 Ford Mustang

A few weeks ago, I had to drive across town to pick up my sister from work (ironically, she was having a car problem. ) I stopped for an accident, and I noticed the headlights getting dimmer and dimmer. I looked at the battery gauge and saw that the car was dying. Since I was in the middle lane, I kind of of panicked and shut the car off, figuring I could let it sit without running for a couple of minutes to give it a chance to get it's act together. When I saw the traffic begin to inch forward I tried to turn the car back on, but it wouldn't budge. I didn't even hear the starter turn. It was completely dead! I put on my blinkers, and those fizzled out within minutes, too. So I was, stuck on the middle lane of the highway with a dead battery. I tried to get someone's attention for help but people just stared and inched forward with the rest of the traffic. The right lane and my lane started moving at regular speed so now there were cars and semis whizzing behind and past me. Finally some guy stepped out and helped me push my car to the edge of the road. He tried to jump it but it still wouldn't start. A cop pulled up and helped me jump the car with some heavy duty cables. This was weeks ago and the car has been fine since. I had the alternator, battery and everything else checked at the shop right away and nothing was wrong. I've already had my alternator replaced twice since I bought the car two years ago, the last time being in February. So it just gives out when it feels like it. I even had the belt replaced a few months ago, so that's new too. It's does this periodically and only when I'm driving, and nobody can figure out why.