My brick has 280k miles. About two weeks ago, I noticed the following. Every time I start the engine, about five minutes later, the temperature gauge creeps up to just below the red mark; this lasts one minute regardless as to whether I turn on my auxiliary electric fan or the heater full blast. Only cool air comes out of the heater. The upper radiator hose is warm to touch and the coolant overflow tank is overflowing with coolant; about a quart of coolant is lost each time. This overheating occurs every time, whether I am driving on a hill or level surface. Last week, I changed the overflow cap and thermostat (installed 180' Beck-Arnley) and flushed the radiator (which is only 1 year old). I also squeezed and squeezed the upper radiator hose and turned the heater on full blast when I was flushing the radiator. The flame trap is clear. No oil is in the coolant (I changed the oil this afternoon). No coolant is in the dipstick or on the spark plugs. No coolant is leaking anywhere - except out of the overflow cap. Timing and idle speed are correct. No white smoke or water comes out of the exhaust. When the engine is off, coolant pressure is constant. The gas gauge appears to be working well.

Thanks in advance