1997 Buick Park Avenue engine runs hot without heater on

My park ave ultra runs hot (over 200 up to 216) when I am doing stop and go driving unless I turn on the heat/defrost. It runs normal on the highway.I have replaced the thermostat, the upper and lower hoses, had the system flushed out at the buick dealership, checked for holes or leaks, and had it looked at by a mechanic. What I want to know is if anybody knows why this is happening and what can be done?