Overheating at idle may be an airflow problem through the radiator. Are you running a fan shroud? Also, what fan setup, and how much distance from the radiator.
I have read that an air pocket in the engine can cause problems, usually the fix is to drill a small hole in the thermostat so the radiator fluid can push the air out of the engine when the engine is cool (before the thermostat opens.)
Also, make sure the lower radiator hose is not restricting coolant flow (not kinked or getting sucked closed when the engine is running.)