Yea it does sound like your fans arent working properly. At idle speed or city driving the fans should be on to cool your engine down. At highway speeds there are no use for fans since the speed that you are driving at is sufficient enough to cool the engine. Also when the thermostat was replaced did you make a small hole at the top of it? I know most part stores dont sell them with the hole already there. This allows air in the system to escape, preventing air pockets in the coolant.