you sure its the coil? more likely the control module or dist. pickup. both can do the same thing you said yours did. these coils rarely don't work. the spark they produce starts to weaken over the years. but it should still work. the wires on the pickup break often. as the advance moves the wire inside breaks contact. the modules are also known to be bad, most parts stores check them. but they often fail when they get warm and not when they are cold. sometimes its hit or miss where they don't work. wait till it happens again, even if you leave it idling in your yard.
Does the coil get hot? I have been fighting with the coil on my bronco for a while, it gets super hot and quits working. After it cools down a little, it will run again only for a few minutes. Depending on your ignition system, you should have a ballast resistor or resistance wire. Check your voltage at the coil while it is running, it should be around 9 volts.
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