Triumph Repair: engine overheating when idling, overheat radiator

Question
I have a 68 tr250. The engine starts getting more hot, and the temp gauge rises to hot when idling only. This does not happen when the car is being driven. I recently replaced the radiator cap, thermostat, and temp regulator. Warming it up recently for around 10-15 minutes, it overheated.What could be the problem?

Answer
Not enough water is flowing given the heat generated by the engine.   The problems could be:

Generating too much heat
-TIming too far advanced at idle
-Mixture too lean at idle
-small crack in coolant jacket that is allowing a small level of combustion gases to enter the coolant.   

Not enough coolant flow
-Water pump impellor problem
-Collapsed radiator hose
-Blockage in radiator itself

From my experience with this problem, the issue has typically been the radiator needs to be flushed, if everything else checks out OK.   Another "patch" is to put an electric fan that allows you to increase the airflow better than the mechanical fan on the engine, and remove the mechanical fan.