What's causing idling issues with my Ford Taurus?

[A_Mang]

I have a 2002 Ford Taurus SEL (24 doch) with 76,000 miles and an automatic trans. Last week, after driving all day without any issues, I started the car which immediately began to idle very low (200-300rpms) When I put the car into gear, it quietly died. I started the engine again but had the exact same problem. This happened about 5 or 6 times before I got the engine to stay on. Somehow I made it home but every time I was coming to a stop, the car would die. Once the car got past 5 or 10 mph everything was fine. The next day (and ever since then) my car has run more or less fine. The thing that worries me is that my engine now idles at about 650 rpms when I'm stopped and the needle seems to jump around slightly if I turn the wheel a little. After your basic 30 minute web research session, I think I'm dealing with one of a couple things: An air flow problem (the air filter was clean as a whistle), a fuel injector problem, or an electrical sensor issue. Is 650 to low to idle at? Could the problem just have been that day and now I shouldn't worry? Does anyone have any ideas what could be causing this? If I'm way off base with my research, please tell me! Thanks.