[decooney]

Sparks,

in addition to resolving any float or sticky needle/seat issues first...

experienced the exact same situation with a friends newly built car last year. It would run, stop, start right back up... go for an hour... then stop again for no reason. Turn the key two times, and it would start. No consistency. We tore the carb apart, played with the floats, and it seemed like it was flooding. Had to be fuel, right? We were sure it was the carb, until running out of choices.

The Result:
a bad pickup coil in the brand new MSD distributor. Ordered a new one from Summit for $17. Cured, no problems since. Its an easy check. Check for recommended resistance across the two wires in the plug - per you mfg specs if you are running this type of distributor / pickup setup.