5.9 gas cooling problem

Having a cooling problem  I just bought this truck
the radiator was sepperated from the plastic side
tanks to the cooling tubes I've replaced with a
used one the clutch fan is new and the water
pump was replaced and the thermostat 194° put it
a pan heated up on stove to make sure it worked    
I can let idle for 40 minutes as I'm filling water
making sure all the air is out with the a.c. on and
temp climbed to 210° after an hour of run time all
and all with everything buttoned up  I can live with
that but as soon as run the truck 70 mph it gets
hot 230-250 turn off the a.c and it comes back
down to 230 and seems to stay but if you push the
motor harder it gets hotter as I go slower setting in
traffic/stop lights it cools down to 210-220 I just
don't get it. It seems to be backwards it should be
over heating with the a.c. on idling other than
rolling down the road with more air and water
movement any help would be great info