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It seems to be better on the interstate at highway speed, but once slowed down in traffic is when the vapor lock situation starts....As far as the fan goes-----I have a 7 blade big pitch after market direct drive fan-----it pulls air through the radiator like a vacuum cleaner.....

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Ok what's the size of the radiator? I had a similar deal on my 64 Dodge...I went from a 2 core to a 3 core and never had a problem again. It got so hot once on a humid,hot summer night,it vapor locked shut-off & melted the shifter cable and was stuck in high gear...
Example if your using a small block radiator w/22 inch radiator support it will stay hot down the highway but with alot of air it helps get rid of heat under the hood. But then it gets really hot in traffic due to the low volume of water it has to cycle to keep cool even if the fan works great...

EDIT...Sorry I saw your post the one about the new 5 core it that's the case the top and bottom tanks must be huge...If they are stock tanks don't know how that would work...