Ford F-150 Immediately Stalls

Hi Folks, My 2000 Ford F-150 is having problems when I try to start it...sometimes. Most of the time, it starts fine. However, occasionally it appears to start normally then stalls immediately. If I give it gas before it stalls, the engine will run as long as I'm giving it gas, but as soon as I take my foot off the pedal, it stalls. Usually, after multiple tries, it will eventually start. Also, it sometimes seems to help if I floor the gas pedal for a second. Any ideas what's going on? I haven't noticed any patterns as to when this happens. Today it is cold and rainy, and it is happening. I know that it isn't the cold that is causing it, because this happened to me a few times over the summer. Could be the rain, though. Travis