1993 Lexus ES300 OverHeating Problem

My car has 150,000 miles on it and has a factory V6 engine in it. When I start and drive my car, the engine temperature is fine. But if I drive for 20 or so minutes with the A/C on. And then idle my car for 5 minutes. While I am idling, the car overheats. My coolant reservoir is full. I recently had the waterpump, heating coil and thermostats fixed. So my mechanic said something about the radiator fan. I see the fan working when I turn the car on. And I assume that when driving, the wind cools the engine. So any thoughts guys? Do you think after driving that the fan doesn't come back on when I idle? Is that a fan, fan motor, relay or fuse problem? Any help is appreciated.