got a similar problem with my f150, and the problem got WAY worse when i let the tank go dry, so DO NOT, i repeat, DO NOT let the tank run dry. what i have narrowed it down to is the fuel pump. you may have a different problem, however one thing you can actually hear is the fuel pump switching on when you turn the ignition to the on setting (but not cranking it). wait until you hear the thump it makes when turning on, then try starting the car. usually it will start a lot better. two things to try is replacing the fuel filter and replacing any fuel pump fuses/relays. next would be to replace the fuel pump. (as a sidenote, ive had this problem w/ the f150 for 4 1/2 years and have yet to replace the fuel pump)

**your problem could still be spark and/or oxygen as hangwire mentioned, so my diagnosis may not apply here**

 

Ok so i picked it up from the dealership and they told me Ford's saying it's the alternator which I was kinda surprised by, but now i have a diagnostic recorder attached to my computer under the steering wheel, to record some data next time the stang doesn't start.