1997 Chevy Lumina Hard starting

I'm having an issue with my Lumina. From a cold start it will start just fine and then run continuously for 6 hours or so (I'm a rural letter carrier in northeast Indiana) but after the day is finished and the car is parked it seldom starts without pushing the gas pedal to the floor and then if it starts the idle is terrible with the engine missing alot or stalling. Sometimes it will not start at all. Any ideas or first course of action?