2003 Ford Mustang Problems Starting

My car will turn over when I try to start it, but will die unless I push the accelerator. Once I hold the accelerator long enough, the car will rev up itself and then idle fine but it is just getting it to idle on its own at first is the problem. Any ideas what might be wrong? It does not do this all the time however, usually only when I am first trying to start it after it has sat all night