After driving with the engine warmed up to normal temp (around 200), when i get stuck in traffic or at stop lights my engine temp slowly inches up to the 230-250+ range. I've had a new thermostat installed and had the coolant flushed, but it still does it. The only thing that seems to lower the temp (while stuck in traffic or idling) is turning on the heat full blast.
Does anyone have any ideas what could cause this? Probably an expensive fix....