68 Valiant dies and runs lumpy, then suddenly runs fine.

I had the same exact symptoms. I thought it was fuel related so I disconnected the fuel line from the gas tank to the fuel pump, I blew aair back down the hose to clear it up and clear any dirt. Open your gas cap and have a friend check if there's any airflow out the tank while blowing confirming that the hoses are not an issue. it worked for me!