Just a thought - were all the other radiators you tried second hand? Perhaps they have been partially blocked - the first thing I did when we got the 1200 was put a new recore in it. (I hate overheating, so the first thing I do when I buy a car is thoroughly refresh the cooling system).

Otherwise I'd be having a good hard look at the head gasket. If you go to a radiator place, they can do a litmus test to see whether there's exhaust gas in the coolant.