What causes what seems like vapor lock?

First let me say that I read some of the other responses you gave and I saw that you said that fuel injection does not vapor lock. That said, it sure as heck seem like that is what is going on. About 3 years ago I was driving in the city, very hot day, normal city traffic. The car started to miss really badly. Wouldn't accelerate much. I made it to my destination, parked and could hear this noise like something bubbling. Got out and opened my trunk, which was empty, and realized that the gas in the tank was literally bubbling. We let the car cool down for a while and it ran OK. This has not happened since until today. I was driving on the interstate, it hiccuped, was fine, then about 20 more miles it started missing, losing acceleration, getting acceleration back. I had about 1/2 a tank of gas (I don't remember how much the 1st time). I took exit 2 miles up rd, barely made it to gas pump, filled up which cooled off the gas in the tank, and it was fine again. Started right up and not even one more hiccup, even climbing in elevation as I was. Do you know what causes this? Are there parts on the car that could contribute? Is there some type of gasoline additive to prevent this?