GreenD16Y8

hey guys my civic is overheating only when i get off the highway it stays cool cuz i do a lot of highway driving. I have done a lot of diagnosing, and still cant find the problem. maybe someone can help. I have change the thermostat 3 times, new radiator, engine coolant temp sensor, fan switch, new hoses, flushed coolant, new fan, and yes the fan does turn on but the car still continues to overheat. the only part i havent changed or checked is the sending unit that gives the temp reading. There are no leaks coming from the head and i am not burning anything harsh in my exhaust. I really hope someone can help me out. By the way i am a bmw technician, so i pretty much know what i am doing, but maybe someone has come around this situation before. So if you have any info please help me out thank you very much.