[Dirty Harry]

Is there any chance that you have the head gaskets backwards, blocking a coolant passage? Is one of the radiator hoses (typically the lower one) collapsing? Did you try drilling a small hole in the flange of the thermostat and orienting that hole to the top? Did you try having the front of the car elevated when you filled the radiator? Has the distributor gotten loose and rotated, causing the ignition timing to be 'way off? If you changed the engine from v-belts to serpentine belts, did you make sure the water pump isn't turning the opposite direction? Has a baffle gotten loose inside a muffler, resulting in excessive back pressure? When the car is up to temp, use an infrared thermometer to compare the temps of both radiator hoses. These are just a few "outside the box" ideas to possibly help solve a 13 year problem.