2001 Pontiac Sunfire engine runs hot

Over a month ago my car started to run hot. My mechanic replaced the thermostat and still ran hot. Then I brought it to the ponitac dealer and they said that it could be the head gasket. I took it back to my mechanic and he stated that it was hard to believe since the car hasn't lost any fluid. So he replaced the water pump and radiator. Car still ran very hot. 5 days ago a new head gasket was put in and still to my mechanics dismay the car ran very hot once again. So what else could it be? Any suggestions would be appreciated. This is a very good running car even when the car is pinned on hot. Thank You