So somewhere you are loosing power and if it does it just sitting it is probably something heating up rather than a loose connection that may get bounced while driving. I would check the ignition switch and the wiring to it to start with, also the junction block on the fire wall. The tend to corrode. If it was a points ignition before and you installed an HEI you need to eliminate the resistor wire as the HEI requires full battery voltage.
Artificial Intelligence is no match for Natural Stupidity