Had the same problem with overheating only during the hot summer months when going uphill or longer freeway rides.
The quickest way to cool it down again was going downhill in neutral.
I went through two old radiators. The last one was better and looked very new on the outside.
However most all of the old honey comb single cores are partially or over 50% clogged and corroded inside.
Even concrete cleaner over night couldn't penetrate although it appeared that it really did its job.
I went to 3 radiator shops to hear their advice and believe to have found the best solution.
The cost to have my radiator recored or buying a newly build one was both $280.
It's always better to keep your original overflow tank, frame and possibly bottom, if it is in good condition or just needs sandblasting.
I got a 3 row modern core from Craig, which is the only way to go.
This summer I can stand in traffic even right after a long freeway ride with no overheating problems.
I also got an additional overflow tank just to be sure.
Bottom line, if you live in the southern states and still have an original honey comb core, have it recored and restored in any case - it is clogged even if you think it isn't.
The temperature sensor doesn't show how hot the cylinder walls really are and I was told that the old sixes were notorious for cracked heads from overheating.
While we are at it....
I am not affiliated with this company, but if you like your engine, please never use a single drop of water ever again.
This alternative really works and is not just another scheme:
NPG+ No more worries about corrosion and water hardness and keeps your cooling system efficient for the life of the engine.
I am usually skeptical with all these new snakeoil additives, but this stuff is what every engine would put on its wishlist, no doubt.
All the bliss has it's cost. Not cheap but it's a one time fluid replacement and you never have to worry about your cooling system again.
Even the hoses require less frequent attention.
