1991 over heating

I would like to know whats causing my car to over heat just bought it. The lady i got it from had the water pump done, Then i just replaced the thermostat this mourning an it still gets hot.on th e drive home from were i bought the car is about 45 to 50 mins away.The whole time on the freeway it didnt get hot but as soon as we got to the city wheres there stop an go traffic it gets hot.if you have any advise that would help, i would apreciate it very much. Thank you an have a great day. Roy