Quote: I had a 69 Dart slant six that would do the same thing. I actually used it to tow my Harley down from Illiniose to Daytona Beach for the 50th Bike Week and had to stop every 100 miles or so to let it cool down, it got worse of course because I was towing a single axle trailer. I tried everything on the way down, then I got mad and got ready to throw the radiator cap across the highway! I took a good look at it and the brass seal was cracked it three places! I walked across the street to an auto parts store, bought a new cap, and never had another issue. Basically there was not enough pressure in the system to keep the coolant flowing.
Rhino, the coolant flowing wasn't the issue. Every pound of pressure increases the boiling point or temp at which it will boil. I can't remember exactly how much but that's what the cap does. And of course keep coolant in. LOL
I tried everything on the way down, then I got mad and got ready to throw the radiator cap across the highway! 
I took a good look at it and the brass seal was cracked it three places! 
I walked across the street to an auto parts store, bought a new cap, and never had another issue. 
Basically there was not enough pressure in the system to keep the coolant flowing. 
