Buick LeSabre 2000 can't start

I had problem to start the car in the evening. It cranked but did not start. I pump the gas pedal, it started. But I can start it smoothly just now.
What's the possible problem? Ignition wire? Spark plug? or gas pump or low gas level (1/8)? Thank you.

How long had the vehicle been sitting before the hard start and how long for the easy start.
If you depressed the accelerator pedal and the vehicle started, check and clean your Idle Air Control valve.
When you depressed the accelerator pedal you were increasing the amount of air available to the engine and leaning out the air/fuel ratio.
Speaking of what was the ambient temperature during each start ?
Is the vehicle idling smoothly or roughly ?

Just some thoughts

8 hours before hard start, around 50F, pretty humid weather in Louisiana.
1 hour before easy start. around 50F.
Vehicle is idling smoothly.
It happened once in middle night, but I could start normally 10 minutes after the failed start.
Check engine light is off


How long had the vehicle been sitting before the hard start and how long for the easy start.
If you depressed the accelerator pedal and the vehicle started, check and clean your Idle Air Control valve.
When you depressed the accelerator pedal you were increasing the amount of air available to the engine and leaning out the air/fuel ratio.
Speaking of what was the ambient temperature during each start ?
Is the vehicle idling smoothly or roughly ?

Just some thoughts

A couple of things that can be done:

Check and clean the IAC valve:http://i481.photobucket.com/albums/rr175/discnik/IAC_LeSabre.jpg

Use carb cleaner and thouroughly clean all the ports ,throttle throat and plate.

See if helps.