I would see if an infared temperature sensor would help diagnose the problem to a faulty sensor or a bad core to the radiator, or a bad thermostat. A good one would be one from Raytek called the Raytek MT4U MiniTemp No-Contact Thermometer with Laser Sighting. They can be found on Amazon.com or any auto parts store for less than $100. You can tell if the radiator is blocked as there will be hot and cool spots on it. You can also tell if the block is a lot hotter than the radiator, etc.... These things are really good diagonostic tools that have come down in price substantially since they were first intoduced.