Dying Dart after being warmed up.

after about 15-20 of driving she'll start to sputter out and die when left to idle. Like I'm cruising and it seems to run fine, but once I get to a stop light it dies. To start it again I have to turn the key and keep the RPM's up while in neutral. I'll take off it stays alive at cruising speeds, then dies again once the RPM's get to idle.
This is what makes me think idle, air/fuel or vacuum leak.