Alan,
Your variable running temperature related to road speed suggests that your cooling system isn't pressurising correctly. It's perhaps worth looking at the expansion tank cap first.
By the way, your account of the cooling system's operation is basically correct except that in the case of the 4 and 6 cylinder 75/ZT engines, the OEM themostat is located in the return from the radiator, not in the hot flow to it.
Simon
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"Whatever is rightly done, however humble, is noble."
Sir Henry Royce.
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