Does the fan shroud have air baffles or reliefs? If it's a flat shroud that encapsulates a single fan, it's actually probably blocking airflow at those speeds.
The shroud needs to have reliefs in it to allow air to flow through the radiator. At speed, the fan itself may be acting as a restriction as a result.
Other possibilities include a lower rad hose that is collapsing at somewhat higher rpms and not allowing proper coolant flow.
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-Jason
1969 Pontiac Firebird
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