on your starter motor you have two wires one main power feed and the other one which will be the problem is connected to the starter motor solenoid. first time you start the car or if its left for a while it will start but if you stop then try to start it straight away it wont start. i bet the stud the wire connected onto is loose the heat from the exhaust down pipe always causes this to happen. only remedy for this is to somehow weld the stud or buy a new starter motor and heat wrap the down pipe.