Does it crank good all the time and just not start, or does it crank slow or not at all?
My first thought would be corroded or loose wires to the starter /battery/ ground.
Second involves a question, does it start fine when the engine is cold, and hard if it is hot, as in recently parked? That woudl tell me possibly worn out starter.
[This message has been edited by 2.5 (edited 08-07-2014).]