If it was anything major, you'd think it would overheat well before an hour. Flow through the radiator at operating temperature indicates that the thermostat is open, the water pump is pumping, and the radiator has some flow.
It kind of sounds like there might be insufficient flow through the radiator to remove all the heat. Make sure that you have the correct coolant ratio and if possible, take several readings with an infared thermometer to see if there are any hot spots, or cold spots in the radiator. Sometimes mineral scale will plug up some of the cores which basically reduces the size of the radiator. Taking IR readings will also tell if the gauge is giving erroneous readings.
Of course the first thing to check is if the coolant is brown / rusty or just looks bad, then drain and flush forwards and backwards a few times until it runs clear.