Staring Problems

[Doom's Day Custom]

I have a 99' ford explorer with a 5.0l... I have done a full tune and post what up below what I replaced so far.. But my troubles are the the motors turns but it has a hard time starting up and starter has to turn the motor for extended times in order for the motor to start up... This does not always happen all the time and I am lost on what it is ????



Parts replaced -
Spark plugs (Motorcraft)
fuel filter
Spark plug wires
pvc valve
Battery
oil and filter
air filter
water pump
Radiator
thermostat