98 cavalier overheating. please help

My cavalier is running very very warm. i flushed the radiator and put in new antifreeze. at idle it does not overheat, but runs near the Hot line. once i start driving, around town it stays about the same, when i get on the highway after about a mile it starts to rise to hot. after i stop the car to check the coolant again, i can hear a popping in the overflow tank. i figured there was just air in the system, so i ran it and kept putting more coolant in. is it possible that their is a blown head gasket and air is getting in the system?? any help would be great.

thanks
mike

. oh i forgot to mention, i also removed the thermostat and placed in a pan of hot water to make sure it was opening and it did. so i reinstalled that. thanks

try flushing your heater core

with engine warmed up radiator cap off check for curculation.you should be able to tell by reving the throttle.if not possible water pump.also make sure nothing is blocking radiator(paper/plastic bag)if it only overheats at highway speeds it sounds like improper coolant/air flow.check to make sure you dont have callapsed hose some where.

. oh i forgot to mention, i also removed the thermostat and placed in a pan of hot water to make sure it was opening and it did. so i reinstalled that. thanks

did you make sure that "hot water" was the exact same temperature the thermostat is rated at when it opened? what i mean is if its a 180 degree thermostat, do you KNOW it opened right at 180 degrees? or could it have opened at 200, far after it was supposed to.....?