Overheating at idle

Temperature is fine as long as am driving, with A/C or vent air on or off until I am at idle for more than ten minutes then it gets heated passed half way mark no matter what air is on. Turning the heater on resolves this and it will, cool back down slowly. I just replaced the thermostat, today thinking that was it, had A/C system repaired/fixed about two years ago, and now, literally twenty minutes ago, no cold air! Could just need Freon topped off, but is the radiator a possibility, being clogged or rusted/damaged? Is the heater is serving as radiator/heat escape in a way? And how would I go about checking it, or any other possible causes? Much appreciated! A.D.