Hi;
The Pontiac dealership dosen't know either as they cannot get it to do it while they have it. They think it might be the passkey security system. They recommended that the next time it does this to turn it off, remove the key, re-insert the key and turn to the on position (do not crank) and leave it for 10 minutes, then turn it off, remove the key and then re-insert the key and try to start it. This would reset the passkey system to the key.
Earl