At that point, you need to replace the radiator. Going aftermarket is good in the case of these radiators, so you can get away from plastic...it's a great idea. It sounds like your thermostat is ok, the car is running where it should be now. If you hear the fans not click on, or see the temp gauge reading cooler than normal (near the C mark, or under 1/4 of the way up the gauge), that would be a time to check the thermostat.