Need help with an overheating issue in my wife's 2007 Chevy Equinox.  The car overheats when idle, such as waiting at a stop light, but then cools off once the vehicle starts moving again.  Also the heater inside the cab does not work as well, except for an intermittent period of heat the goes away quickly.  I have replaced the thermostat and serpentine belt.  Have also had the coolant system flushed and pressure tested through Firestone, how the coolant inside the reservoir does not appear to be moving/cycling.  Cooling system is a closed system and has no direct fill access to radiator.  I have no engine fault codes and there is no smoke from the exhaust.  Any assistance would be greatly appreciated.  Thank you in advance.