In the 80's(1983) my sister had a 1972 Hornet straight six auto, and every once in a while I would have to rescue her because the car was stalled. I would get there, and the distributer cap was off. After four or five times, I started putting extras in the trunk. it was the strangest thing, then once it did it in my driveway, and i heard it pop off.
Apparently, some fuel vapor was finding it's way to the vacuum port on the carb for the vacuum advance. so upon acceleration the vapor would be sucked into the distributer past the advance, and explode from the points spark, blowing the cap off!
Only on a Rambler,,,,,