engine won't keep running
 

67 Mustang 302
took the car for a drive today to attend a parade.
car ran fine to downtown, but wouldn't start once parade started lining up.

car starts for a split second and dies, hold the key turned and it'll keep running.
has spark on number 1 plug, gets fuel, new battery, and new alternator about 6 months ago. Installed new ignition switch, new starter.

any ideas?