[undy]

It could be an ignition module (not knowing which distributor your using) or a MSD box intermittent failure. You could have a resistor/diode/etc that's heating up and cutting out. As it cools the system starts working.

Carry a known-to-be-good spare spark plug and pull a wire when she dies to see if you've spark. Another thing... as soon as she dies, pull her over to the side of the road, pull a spark plug to see if it's wet.. wet plug = no spark and a dry plug = no fuel..

Dave