hot running dart

Hello!!!
I have a 1971 Duster 340 that I am having a problem with.... Running down the street, I am fine! Temp will stay steady at 180-185... if I have to stop at a traffic light or get in heavy traffic, stay at stop sign too long, etc... it will pop up on 200-210 everytime!! Motor (340) has been gone through, it is good! I have tried 3 different radiators, 3 different fans, 3 different thermostats, even changed the temp gauge several times.... cannot change the problem!
PLEASE...... any suggestions?????????