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Hello and welcome to just answer. This is probably not a fan clutch problem because when you are doing 65-70, there should be sufficient air flow through the radiator. If the fan clutch was failing, then your overheating problem would happen during stop and go type driving. This sounds more to me like a coolant flow problem, like a partially plugged radiator. It may also be a cylinder head gasket issue, but if you are not loosing coolant then it is probably not that. I recommend having the radiator flow tested and replaced if necessary. Also make sure that there are no blockages in the radiator fins preventing proper air flow.
I hope this information helps, and let me know if you need further assistance with this.