just as JMD says, you need to diagnose once it happens not hours later when it works again.
As you say it seems to happen when you're waiting in line, so maybe the same will happen in front of your house if you give it a try.

i once had an issue with a new pertronics which could be related to your problem.
you have a vacuum advance on the front of the distributor. try rattling that. it should be mounted solid to the distributor
When i got mine it was a bit loose, so I could rattle it loosing ground to the ignitor module in there. took me ages to figure that one.
Apart from that my money would be as well on fuel issue.

I don't know ablout mustangs and how they air the tanks, but in some occasions (other cars) I've seen that while the fuelpump is sucking the tank empty it would create a vacuum in the tank. getting stronger and stronger so finally the pump couldn't overcome it anymore. Having the car sitting around helped or even simpler just opening the gas cap and closing it again (to let air in). You usually hear a swuuuuuuush when that's the case when you open the gas cap

i'd say your list is getting longer and longer.

when you bring it to a garage they probably will not be able to help you when the issue is not there at the time. So from this try to force your car to do this in front of your house. once you managed that the beast has given up already.

 

Wow, thanks guys... I've got plenty of work to do, don't I... great idea at causing the problem in front of my house!

JMD... you asked if I had removed the resistor, I didn't. The pertronix instructions mentioned that it's not necessary, so I didn't even think about it... But could that cause this problem when it only happens when the engine gets hot and is in slow traffic?

I've got a hunch, (and with all your help), that my problem is with the fuel system. The fuel line runs right over my intake manifold to the carb... seems like a great place to get hot & cause vapor lock. Where is a good alternative for running the fuel line, or where should it run?

 

I have heard that the Pertronix II instructions say that the resistor can remain, but I want to say that some posters on the forum have had issues that were resolved when the resistor was removed. (Heat will increase electrical resistance.)

While vapor lock may be your issue, I do not think that the line from the fuel pump to the carb is yourproblem.

Vaporizedfuelactually inside ofthe fuel pump causesvapor-lock because the fuel pump looses it's ability to pump when it becomes filled with gas (not gasoline but gas as invapor). If vaporization occurs beyond thefuel pumpit is usually easily expelled past the needle valve and is not an issue.

For a vapor-lock issue you want to look forheating of the fuelprior to it entering the fuel pump, not after it leaves. (for carburated cars anyway)

When the (expected) seasonal temperatures are considered, In my opinion you are not experiencing vapor lock unless there is a significantsection (like at least afoot or two) of the fuel line that runs in very close proximity(an inch or less) to an exhaust tube.

I just do not think it has been hot enough yet for you to have an issue with general vapor lock, but I am more than ready to admit that I might be wrong... ,,, but I don't think so...