/6 overheating need help.

So I just fixed the a/c in my car and figured I'd finally put the 16 inch electric fan on my car I got with my 2 row champion radiator and now it overheats fairly quickly. I was running a direct drive fan with shroud and the car never saw past 180 even with it being 100+ degrees outside right now. I have a fairly new water pump and thermostat in. Now here comes the weird part it only overheats while I'm driving and when I'm stopped it drops fairly quickly. I have an aftermarket temperature gauge and electric setup with it. I even let the car idle in my driveway till it hit 200 turned on the fan and it cooled to about 170 but when I drive the car it just warms up. Its 1973 dodge dart 225 I even put water wetter and nothing. Please help!