Flushing the system out with a hose when changing coolant is a good idea, but avoid harsh chemicals. They can cause more problems than they solve. The best plan is to disconnect the radiator hoses and remove the thermostat. This way you can get a good rate of flow and wash out any sediment, loose debris or pieces of scale that are not firmly adhered. Flush out the radiator this way too, and if you want to go the whole nine yards, disconnect the heater hoses and flush the heater core too. Use distilled water and the correct type of coolant when refilling. Use a new thermostat gasket.