overheating 1996 sunfire why???

hi i have a 1996 sunfire with a overheating problem . first of all i changed the head gasket and after that i noticed that when i went on the highway the temperature of my car went down so i changed the thermostat and also vacuum the air out of the hoses( need to do that for the heater to work) after i done that i notice my car overheats why?