Duraspark II problem
Check the reluctor in the magnetic pickup on the distributor. I had exactly the same problem - it would start fine, then run until it was warm, then cut out as if it had been shut off. There was no spark until the engine had cooled off, then it would repeat the same scenario again. The rest of my ignition is a Jacobs Pro-Street.
Genuine original one-of-a-kind 1977 Hurst Granada GT