I hope that you are not getting your repair advice from a mechanic. The parts you have replaced may have been old and degraded, but they are not likely the culprits for the symptom you describe.
If the car gets hot when it is stopped, but cools back down at highway speed, the problem is almost certainly the lack of forced air through the radiator. Does your electric fan in front of the radiator come on when the car is stopped and starts to get hot? If not, it could be the fan, its relay, or its water temperature sensor.
When you replaced the radiator, did you check the air conditioning condenser to make sure that the fins were not blocked by bugs, plant debris, or pet hair?