first and foremost...don't ever open the radiator when the car overheats...you'll burn your face off...literally! Does sound like the thermostat. I always check the radiator hose when a car overheats, if its soft and you can squeeze it somewhat it should mean the thermostat is working. If its hard and does not compress much usually the thermostat is done. Im no expert but always has worked for me. And don't open the radiator cap when the car overheats. I would change the thermostat , not too expensive. If you live in a warm climate ...some people remove it all together. Good luck.