Something similar happened to me recently with my Corvair. I replaced all the ignition components, started it up and it ran great. Jump in the car for a test drive and after about 3 miles, the engine died at a stop sign. Would not restart. I assumed I was out of gas (I was on my way to fill it up). My wife brought me the gas can, added a couple gallons and the car started right up. Filled the tank and drove around some more, then same thing happened...just died at a stop sign and would not restart.

Eventually tracked it down to a BOOB (Bad Out Of Box) condenser or coil (replaced both at same time). Once one of those two components got hot, the spark went weak and engine would not run/start.

So, maybe you have a similar issue with your ignition components.