Vapor lock

That is possible. What valve do the tuning specs call for and have you experimented going up or down, or putting a block off in place of the valve. Years back when I ran a 340/6 4 speed car on the street, it was a constant tuning event. This was because the car was my DD, but was raced on the weekends. I know technology has come a long way since then, I'm just adding .02.