1999 Pontiac Grand Am Trouble starting

Engine Performance problem
1999 Pontiac Grand Am 6 cyl Front Wheel Drive Automatic >130K miles

After the car sits over an extended period of time (overnight/commuter parking) it will not start. The starter turns the engine over but it will not run. Also happens after it is driven and you try to start it after turning it off for several minutes. Occasionally when starting it, the engine stumbles and won't continue running without flicking the accelerator. On other times the engine starts fine but stays at around 2K rpm even after warming up.

Had it towed twice to the mechanic and it has started flawlessly. They held it over night on both occasions and again it started fine according to them.

The battery is new as are the air and fuel filter. The plugs were chaged 2 mos. Ago and the wires 6 months ago. According to the mechaic the fuel pres. Is at 55psi. Once running, it runs fine with no stumbling or hesitation. No codes or check engine light.