2002 Ford Explorer car keeps running hot

I have changed the thermostat 2x replaced the thermostat housing top and bottom, replace the thermostat cover , had the radiator flushed and replaced the radiator cap. I have also taking it to the shop and they can find anything. I have notice though it only runs hot when I am in a lot of stop and go traffic. I have been checking the cooliant level and it still normal so what could the possible problem be?
Thanking you in advance for your help,
Sincerely,
Sophia