The fuel pump could be weak. if the tank is more than a quarter full there is enough pressure from the tank to keep the fuel flowing to the pump, if less gas intank the pump cannot pull itthrough the line. Another thing that can cause this problem is a pin hole in the gas line. An old Mustang I had started very hard when I first got it. Once running it was fine. It never leaked gas either. Eventually I replaced the fuel line and discovered that the old line had a pinhole where it went up over the axle. When the car was stopped thefuel would drain out of the line back to the tank and the pump couldn't pull it back over the hump, but once started it would run fine.