Setting adjustable sensor for fan

Ok I installed my electric fan setup a few months back and just got my radiator situated. When I initially installed the sensor for the fan, I set it to come on at 185*. ( I have a 180* t-stat installed in my /6 ) .. the issue is that the fan will cycle on and off while the car is moving but will run constantly if I'm stopped. Is this correct? I was under the impression that if I set it correctly it would cycle while sitting in traffic and should not come on if the car is moving. I can tell it's turning on/off by the whine from the stereo :)