98 Cavalier radiator runs hot

I have a 98 Chevy Cavalier, mileage 125600. It has not ran hot very much but I have also kept it from running hot. The ac does not work. I check the coolant once a week and add Dexcool if needed. The temp gauge sits half way. If I am in traffic it goes up quick. I turn the heat settings on high to keep it from over heating and it keeps it down a little. I had the old coolant drained and fresh put in earlier this year. What could be the problem of why it is running warmer than normal and hot?