If it is ok at 60 and not at 70 you most likely have an air flow through the radiator issue. If you park the car and run it at 2500 rpm does it heat up with the fan on. Air flow or an under sized radiator can exhibit the issues you have described. Timing as well but it would not be the first place I would look. You can eliminate it being timing by following F-Birds advise and it wont cost you a dime.
Small changes to the grill area and radiator position can make a huge difference to the airflow.
Also as mentioned the fan does little at highway speed.
Artificial Intelligence is no match for Natural Stupidity