my grand prix will start on occasion and then it wont why

there is a layer of corrosion on the end of the 1/0awg Black battery cable...wirebrush the end of the cable and the spot that it bolts to...assure that your battery is newer than five years old....or get a NEW battery...the 1/0 connection to the engine block (usually near the alternator) needs to be free and clear of corrosion and happy with 300 cold cranking amps~

could be a bad battery connection , bad neutral safety switch, or the sucurity system trying to go bad , does it do it with all the keys that fit the car ?

yes

strange phenomenon...that...your ECU is dependent on steady 12.5 volts with steady unwavering ground....withe every start, the startermotor insists on HIGH amperage, 300 cold cranking amps is minimal...but a blue flame and spark ensues because the electrical connection is of dis-similar metals...at those temps one metal liquifies into the other creating a layer of corrosion....wirebrush this unwanted "semiconductor" layer off the ground connection for a correct ECU operation and NO troubles anymore...god bless~