Overheating issue

I have a 2001 Dodge Caravan that keeps overheating. We have replaced both the thermostat and the radiator. The radiator was purchased used but was inspected by a mechanic and he said it looked to be fine. He put it in and that night & the following day it seemed to be fine but tonight I drove 5 mins to the store & it was almost in the red by the time I got back to my house. What could be causing it to overheat so much? Also, I forgot to mention, he said the head gasket also looked ok, if that helps. Thanks.
Danelle