1992 300e Mercedes Keeps Running Hot & Overheating

I have a '92 300E Mercedes Sedan. It has approx. 196,000 miles on it but I take good care of it all the time and it's been good to me so far. For years, I've had ongoing problems with the cooling system; two very good (seperate) mechanics have tried everything to rectify it. Recently, I've had the radiator replaced, all hoses, the thermostat, the water pump and the sensors. The car STILL runs hot and leaks coolant! Please tell me what else could possibly be the problem. Even my mechanic(s) are stumped on this one. Is there something they're overlooking here? I drive my car everyday and the air conditioner's fine; however, when I'm idling in traffic, I watch my temperature gauge creep upwards. Turning off the AC and turning on the heater brings the temp down a bit, but then I'm burning up inside the car! Help!