1966 GMC Pickup

I have a 1966 GMC pickup with a 305 BIG BLOCK V6, E series. Not a misprint! The transmission is a 3 on the tree. The engine runs good until it gets hot and then violently shakes the whole truck. Then the engine dies until it cools off again, until you come back when it will start up again and run fine. Is this a vapor lock problem, distributor's heat shield problem or what. Does anyone have a guess at this problem?
Thanks a lot