My 1995 Honda Civic LX keeps overheating. PLEASE, PLEASE HELP!!?

So far I have changed the thermostat and have had the fan fixed because there was a delay in turning on. Also, brand spanking new battery, if that makes a difference. While driving on highway it doesn't really overheat too much. It's more while sitting in traffic that the temperature starts to go up. I'm not sure what else could be the problem and when I took it to the mechanic he couldn't figure it out either. There is no leak, when I open the radiator cap there is fluid in the radiator and there is plenty of fluid in the reservoir. I absolutely cannot afford a new car so fixing this issue is an absolute MUST. Please someone help me!!!