ok i will add my 2penceworth, first off the car is an auto, so the autobox chucks extra heat into the radiator and its known worn autoboxes put out a lot of heat :-/
2nd if the oil aint been changed forever then the radiator is having to dissapate all the heat from the engine as the oil just heats up because it has no good heat conductor properties left so the oil cooler aint helping as it should
TB has aready mentioned oil squirters so wont go there as this is also an aid to cooling,
so already the rad is having to cope with at least twice its intended work rate, bottom line is now the rad is full of deposits from trying hard to do the work of three systems,and so is the thermostat and water pump probably
so first a complete service, then as said already stat, w.pump and rad, it should then run properly at the correct temp, this is all a result of improper servicing, and aint knocking you bud you didnt know, but it will pay in the long run to do it all now before you do further damage,i.e. head gasket etc?