Now what?

My 66 Mustang quits running after about 5 or 10 minutes. If I am driving
down the road it restarts and works fine after that. If it happens at a
stop sign, I have to restart it. It starts immediately and works fine after
that. If I park it for a while and start it again the same thing happens.
New wiring harness and ignition switch 4 years ago. Mallory electronic
distributor and new Pertronix coil. Fuel flow and spark are good. 289 4V
with C4 automatic. What should I check next? I hate intermittent problems.
Have lost all faith in this car. Purchased in 1992 and has been great up to
now.