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As a fundamental step, validate the fuel pressure (~7 PSI), draw (>20" Hg), and volume (>1 Qt in 30 seconds of cranking) while the engine is cold. Leave the pressure gauge connected and then monitor the fuel pressure when the symptoms arise. Also, using an HEI spark tester......check the spark color and fullness when cold and then recheck it when the symptoms occur.
Sounds like it might be heat related.