Your fan switch could have also gone bad. You have two fans to cool your engine. One turns on when you are sitting in traffic, it should kick the fan on when the temp hits like 180 degree's. The engine cools down when you turn on the A/C because your other fan is coming on.

Stop and get your fan switch checked.

At least that is what happened to me, my car would only start to heat up when I was in really slow moving traffic or I was stopped.