I used to have a Dodge that would vapor lock in the late spring / early summer. I was told by a mechanic that some brands of gasoline have a "winter" mix and a "summer" mix. Has to do with a change in the volitility of the fuel due to seasonal temperature change. How true all of this is, I don't know. I do know that I started keeping up with where I bought my gas and when I was having trouble and I narrowed it down to Texaco. I stopped using Texaco gasoline during the transitional months of spring and fall and never had any more trouble.