1995 Dodge Intrepid Keeps over-heating

OK, about mid-summer, the car started overheating. If I'd sit in idle, there'd be no warning, and the gauge would shoot straight up and the antifreeze would just shoot straight out the overflow hose. We replaced the water pump, and it overheated a few more times, then just stopped. It seemed to only overheat if we ran the air in town. We were told that we should probably replace the thermostat to stave off any future problems. So we replaced the thermostat and lo and behold, it's overheating worse than it did before. If we sit at a stop light for any length of time, it overheats right then and there. If I run the heater it seems to slow down the pace at which it overheats, but it doesn't stop it. It also has dual fans in the front, and one of them does not seem to be working.