Radiator shroud...

Your fan is the culprit
It's not big enough and is shaped wrong. I see what looks like over 2 inches clearance. Air takes the path of least resistance. At low speed in traffic, the air is just going around the fan tips back to the front side, instead of fresh air being pulled thru that fat-boy rad.
The fan should fill up the shroud to within about a half inch at the minimum, and the hi-attack tips should be at least half way into the shroud. A thermostatic clutch will automatically control the maximum coolant temp, and disconnect the fan at speed,to freewheel, and save you power on the hiway. I highly recommend an all-steel 7-blade fan.
If your fan hits the shroud when under power, you will need to mechanically restrict it, which you probably have already done;or should have done.