My 2003 dodge neon will bubble or boil when turned off, the temperature gauge does not read overheatting,.?

My 2003 dodge neon will bubble or boil when turned off, the temperature gauge does not read overheating. What is the problem? I don't think it's a blown head gasket because I do not have milky oil and nothing is spewing out and I don't think there is a loss of coolant or anything, I went to auto zone and they said it wasn't a blown head gasket, maybe something wrong with thermostat or radiator. idk what is wrong? Please help!