You might need to get the car on a scope to trace down a problem like that. My guess is that it is electrical and probably something thermal related. Most likely a bad coil or a bad part in the ignition box. When it gets hot you have a solder joint opening up or something like that. Very hard to find without a scope or other test equipment. If you have a buddy who has the same ignition system as you maybe you can swap coils and boxes and stuff like that until you run it down. I doubt it is carb related.