"Turns over" and "crank" are the same thing. For intermittent problems the first suspect is a corroded battery cable where it bolts to the starter. You have to pull the insulation back to see the corrosion. There are a lot of other clues and observations that would make the diagnosis easier and faster. Do you hear the starter relay click when you turn the ignition switch to "crank"? What happens to the brightness of the head lights or dome light when trying to crank the engine?