82 Regal Overheating

My 82 Regal has just 10,000 miles on it. It is a V-6 automatic 4.1 engine. Recently it started to reach over 230 degrees in a matter of minutes. Temp gauge is ok as fluid is boiling in resevoir. changed thermostat twice. no noticeable decrease in heat. what is next to check or replace, water pump, fan clutch, radiator??? Help. Thanks