Problem: engine dies after 15 mins. or so Problem: haven't isolated the whats causing this.
Here's what I would do to isolate harness and ign. system . Disconnect wires from + side of coil. Run two wires from battery to + terminal on coil . one wire with an in line ballast resistor . start car and remove solid wire so you don't cook the coil if the engine still dies, you have very little wireing and parts to look at ........ I would unhook your alt. just for the test .
Now if the engine still dies , your down to coil, ecm , dist and carb/fuel . see what happens .
I am thinking we **may** have found the issue when I saw that when the coil voltagge drops below ~4V the idle becomes rough and begins to lower which drops the voltage more and death spirals the motor.
With the fans on and in park the idle is about 800 RPM which keeps the coil voltage at 4.2V which is enough to keep her alive. Once I get below around 750, the alternator isn't keeping enougg voltage which begins this process.
I troubleshot the wiring and I am seeing voltage drop of almost 10V on the OEM ballast I have which is too much for my Blaster coil apparently.
I am guessing with the everything cooled off, I am right on the verge of too weak of a spark but the heat from the motor running may have pushed it over the edge raising resistance in the ballast/wiring.
Not sure why I ran OK before but this started about two months ago. Maybe I was just marginal the entire time and I just stepped over the edge?
I am hoping my theory here is correct as I have about troubleshot the hell outta this ignition system!
If this doesn't work, I will be trying your test along with others...
which is too much for my Blaster coil apparently. 
