Why won't my car start?

I have a '96 Olds Ciera with a 3.1. Now that's it's getting warmer out, it doesn't like to start. I use my car to deliver mail and it will run fine all day until I turn it off and try to start it again. It will turn over but not fire. When it turns over, it isn't smooth. It chugs and sputters and stinks, like a barbecue grill. But it will start with a jump. After I jump start it, it runs rough for a couple minutes. I rev it up a little and it smooths out and runs fine. I've replaced the alternator, battery, and intake air temp sensor. I'm running out of ideas.