car sometimes wont crank

Check fuel pressure to find out if it's the pump, the car continues cranking because the computer controls the cranking, it knows when the engine starts, and it stops cranking when it does. It also prevents you from accidentally engaging the starter while the engine is running. My dad's 03 sierra has the same feature. Check to see if the pump is running, when you first turn the key on, a faint electronic humming should be heard near the gas tank.