That suggests external blockage that is preventing air flow through the radiator. First check for a "butterfly collection" in front of the radiator. If it's clear, feel the fins between the cooling tubes. If they crumble like a rotten chocolate chip cookie, they are corroded and unable to transfer the heat to the air. That problem will get worse the faster you drive and the warmer it gets outside.
Be sure the plastic shroud is in place behind the radiator and the rubber seal is in place along the front edge of the hood. Both of those force air to go through the radiator rather than around it.
Tuesday, April 12th, 2011 AT 7:10 PM