car stalling gas problem?

I have a 2004 grand prix gt and my car sometimes stalls when I go to start it. It isn't rough it just quietly turns off. But if I turn the key again and step on the gas it starts up. When I do that I can hear the gas movibg. It seems to happen when ever I open the gas cap. But once I rev the engine to start it, it usually wont. happen again until I go to fill up the gas again. what could this be I can't find an answer.